It began at 11.07pm one Sunday night, when a police chief with a pot belly stood in a warehouse in the city and began addressing his men; there were around 200 of them, stood in rows and dressed in the black protective clothing of the riot squad.

‘Right, ladies and gentleman,’ he shouted. ‘First of all, can everybody hear me? I don’t intend to use a microphone if I can help it. Right. Welcome to Operation Aeroscope. Many of you, I guess, will be wondering what we are doing here and what all of this is about. I am not going to go into all of the information that surrounds this, only to say that there is information that a group of individuals intent on … erm.’

The senior officer was lost for words. ‘How can I put it? Disrupting a major … erm.’ He scratched his bald head and folded his arms: ‘Erm…’

The crowd of riot officers started laughing.

‘Power station!’ their boss barked. ‘Within the east Midlands area!’

Regaining his composure, the police chief gave a brief rundown of the operation to intercept activists who were planning to disrupt the Ratcliffe-on-Soar power station. ‘There is information that these individuals are at this moment at a location called Iona school, which is off Sneinton Dale. We’re expecting up to 100 individuals at the premises. They are in possession of items that would allow them to disrupt a generating establishment for up to seven days, which is probably unprecedented in this country. They are in possession of lock-on equipment. They are in possession of food which will keep them going for seven days and vehicles to assist them in that.’

He paused and looked out at the crowd of officers. They had been drafted in from across the region, some of them taken off leave during the long Easter weekend in April 2009. It was dawning on the men that they were going to see some action and the excitement showed on their faces.

‘The intention of this operation is to enter their premises, by force, under warrant, and arrest all 100 of them,’ the commanding officer said. ‘Again, pretty unprecedented in this force and you can imagine the planning that has gone into it.’ Both sides – police and activists – had been preparing for this moment for seven months. They were less than an hour away from the crescendo.

The plan to break into the power station was hatched by five eco-activists. Two were undergraduate students, Tom and Penny. Childhood friends, they typified the new crop of articulate, young campaigners pouring into the movement against climate change. By their own admission, they were also a little naïve. Tom, who was 21, was initially surprised that the veteran campaigners wanted to collaborate with two students. ‘You do know we don’t know anything about direct action at all?’ he remembers telling them. ‘Are you sure you want us involved?’ Penny, a 20-year-old, was also taken aback. ‘We were rookies, basically. I was amazed they wanted to work with us. We were in our early 20s and working with seasoned activists and they respected us. We were so excited.’

The other three activists involved in the plot were older and more experienced, and two already had experience of breaking into power stations. But none of the group had ever taken part in a demonstration on the scale of what was now planned. The idea was to quietly recruit more than 100 activists from across the country for a showpiece act of civil disobedience: breaking into the Nottinghamshire station, turning off the generator and occupying the plant for a week. If they pulled it off, this was likely to be the most high-profile direct action against global warming in British history.

Discretion had to be absolute, and the security precautions were a baptism of fire for Tom and Penny, who were quickly exposed to the rigours of ‘activist security’ and counter-espionage. None of the five could confide in anyone outside the group, discuss their plans on the telephone or behave in a way that might raise suspicions among family or friends.

During one of the first meetings, Penny and Tom were told to sit on a bench outside Angel Tube station in north London. They waited there until a man cycled past and gave them an address scribbled on a piece of paper. It was for a terraced house in nearby Finsbury Park. When they arrived, the man on the bike and two other activists were waiting for them. At each meeting, they were given the address of the next rendezvous, and instructed to write down the details on pieces of paper in code. Meetings took place in London, Oxford, Nottingham and Leeds and they chose locations they were confident would not be bugged; church halls, crowded pubs or student houses.

‘We were never meeting in activist-related places,’ says Tom. Fearing their phones might be tracked, they left them switched on in their homes each time they met. If police were monitoring their movements through their phones, they would assume they had been at home. And they always paid for bus and train tickets with cash. For the university students, life was overtaken with some elaborate routines.

Penny, whose job, among other things, was to take notes at meetings, was told to periodically destroy the evidence. One night every few weeks she would wait until the early hours of the morning before stepping outside her Oxford college dorm. When she was sure no one was watching, she hurried along a cobbled path, past a lake, to what she thought was a suitable hiding place. Wrapped in a red overcoat and carrying matches, lighter fluid and a tin pot filled with the incriminating pieces of paper, she kneeled down and started a fire. ‘I never felt so safe as when I had burned everything,’ she says.

Tom had his own curious ritual. In order to make sure his friends did not know he was missing from university in Swansea, he made a point of going out drinking with them the night before he had a planning meeting. They would often end up in a nightclub. The only thing that made Tom stand out was his backpack. At 3am, when his friends were drunk, he would slink off into the darkness and head straight for the train station to catch the night train into London. ‘No one would know I was gone,’ he says.

Two months into the planning, Tom, Penny and the three other activists had reached a crucial stage. They had drawn up detailed plans and studied maps of the power plant. Now it was time to undertake a reconnaissance visit. After weeks of silence and caution, the time had also come to allow a sixth person into their plot. They needed a driver to take them to the power station. It had to be somebody who was totally trusted and experienced, a driver who was familiar with the local area.

‘We were told we were going to get driven up to the power station by this guy called Flash,’ says Tom. His friend Penny says the decision made sense at the time. ‘Mark Kennedy was just known as the man when it came to activist skills,’ she says. ‘He was always up for it and at the centre of everything and just completely trusted and sound.’

At 6am on January 10, after a night at the Sumac Centre in Nottingham, the activists were woken by tapping on the window. They came out of their sleeping bags and opened the curtains to see Kennedy standing outside in the semi-darkness. He looked worried. Despite his efforts, he claimed to have been unable to hire a car. They were going to have to drive to the power station in his personal vehicle. ‘We just thought: we cannot possibly go in Mark Stone’s own car because he is such a prominent activist – he must be known to police,’ says Tom. ‘But we had no choice.’

An hour later, Kennedy’s Octavia estate was parked up in a layby in the village of Thrumpton, 12 miles outside Nottingham. Kennedy, as ever, was in the driving seat. He and another activist were going to drive around the power station to assess the access routes. Meanwhile, Tom, Penny and a third campaigner were tasked with getting as near to the power plant as possible to take photographs with a long-lens camera. They got out of the car and walked south, the silhouettes of cooling towers appearing in the dawn light. They found their way through a muddy field and a small wood before they reached the footpath that circles the reinforced steel fence defending Ratcliffe-on-Soar power station.

‘It was absolutely terrifying because we were just thinking about all the ludicrous security precautions we had taken to not get found out,’ says Penny. ‘And here we are, at the perimeter of the massive power station with the biggest long-lens camera I have ever seen in my life, taking photographs of all of the mechanisms. We were photographing how the site was laid out, the infrastructure, the access routes.’

It did not go to plan. Out of nowhere, two security guards stopped them in their tracks. The guards demanded to know what they were doing. ‘We had a cover story,’ says Penny. ‘We were arts students at Nottingham Trent university and we were doing a project on the intersection of nature and industry. We wanted to get some images of this crazy futuristic architecture and the beauty of the nature around it. We were absolutely terrified so I was just babbling telling this story. Eventually we were basically sent on our way.’

They returned to Thrumpton and waited for Kennedy and the activist in a village hall. When the car arrived, they nervously bundled into the back and headed back to Nottingham. They had been driving for a few minutes when someone in the rear of the vehicle shouted: ‘Shit! Where is the camera?’ The large camera, containing dozens of incriminating images of the power station, was missing. ‘Mark just reversed and turned around the car and said: “OK, stay calm people,”’ says Tom. ‘He said we should retrace our steps and we went back to the village hall and found the camera in the toilets.’

It was a close shave and, for Tom and Penny, perhaps a little embarrassing. They were the youngest in the group of five core organisers, and the least experienced. Mostly, they were treated as equals. However, both of them got the feeling that Kennedy was looking down at them. ‘I was torn between thinking he was really cool – a super-experienced activist – and a bit of a cock,’ says Penny. ‘He was just quite arrogant and definitely clearly enjoyed the company of young women. I felt Mark just had this attitude towards us that we were kids who had bitten off more than we could chew.’

As it turns out, that is more or less what Kennedy did think about the two students. His verdict is contained in the reports he was sending back to his handlers at the NPOIU. The restricted documents, marked ‘secret’, reveal another side to Kennedy: his work as an intelligence source. He made a parting reference to Tom and Penny, describing them merely as ‘youths believed to be based in London’. One NPOIU document, based on Kennedy’s intelligence, states: ‘Youngsters are getting more and more involved with climate change issues on the back of this year’s Climate Camp.’ It adds that they are less of a threat than other activists and ‘just want to save the world from climate change’.

Kennedy’s communications to his bosses provide a revealing insight into the inner workings of the NPOIU. The reconnaissance trip to the power station, for example, is covered in remarkable detail, even recording the incident involving the lost camera. ‘The two girls realised that they had left the telephoto stills camera somewhere,’ Kennedy reports. ‘It apparently contained photographs of strategic points of Ratcliffe power station. They were very worried. I got them to retrace their steps verbally. It transpired that one of the girls had taken it into the toilet in the village hall. We returned to the hall and the camera was retrieved.’

The police operative’s secret reports to his managers reveal the depth of access he had. Remarkably, given all the precautions taken by the five activists, Kennedy knew about the plan to break into the power station before he was even asked to drive them to the plant. Two months before Kennedy was approached by the activists, his superiors had authorised a surveillance operation, stating in internal NPOIU documents that they were aware there was a protest that could ‘damage’ a power station and could cause a ‘severe economic loss to the United Kingdom and have an adverse effect on the public’s feeling of safety and security’. Kennedy was the undercover officer tasked with finding out as much as he could.

A few weeks after he drove the activists to the power station, Kennedy filed another report, this time detailing the precise time and date the protesters planned to occupy the plant. By now he had also established the scale of what they were attempting, telling his bosses that protesters were ‘hoping to recruit 150 people to take part’. This was intelligence gold dust. It enabled the NPOIU to liaise with the Nottinghamshire police force and devise a plan to scupper the protest, exploiting the fact they had Kennedy in a prime position.

The question was: how far should the undercover officer go? Kennedy was asked by the activists to perform his familiar role as driver, carrying a truckload of equipment to the power station on the night of the occupation. But the campaigners also wondered whether he might use his rope expertise to help them with another critical part of their plan: climbing. Getting into the power station was not going to be the hard part. The challenge would be forcing the plant to turn off its generators and prevent it from operating for a week.

The activists had figured out that if they immobilised key parts of the infrastructure, the energy giant E.ON would have no choice but to turn off the coal-burning furnaces for health and safety reasons. They would switch to an emergency supply of gas-produced energy, thereby leading to a huge reduction in carbon emissions for the period during which the plant was occupied. If they could keep the furnaces turned off for seven days, they calculated that they would be preventing the emission of 150,000 tonnes of carbon dioxide into the atmosphere.

That was where the climbers came in. The plan was for a team of climbers to scale one of four chimneys and force the furnaces to be turned off. One activist would abseil down inside the shaft and hang a ‘bat tent’; an expensive, cradle-like contraption used by professionals to suspend themselves beneath vertical cliffs. There was no way E.ON could turn on its furnaces with an environmental campaigner hanging inside one of the chimneys. If things went to plan, and they had sufficient food and water, the climber would be able to stay suspended inside the chimney for a week.

Kennedy was in a Wetherspoon’s pub at Leeds railway station when the elaborate plan involving the climber and the chimney was relayed to him for the first time. It was just two weeks before the Easter protest was due to commence. He was sat drinking a beer with a veteran campaigner, one of the five, including Tom and Penny, who had been organising the direct action from the start. The two men scrutinised aerial images of the chimney stacks and talked about what was required. Kennedy recounted his conversation in one of his NPOIU reports.

‘He started to explain the Ratcliffe action,’ Kennedy wrote of the conversation. ‘He asked if I was prepared to be the main climber on the action.’ The activist who met with Kennedy that day denies he proposed the undercover police officer should take the star role of abseiling into the chimney stack. However, according to Kennedy’s account to his bosses, this was exactly what he was asked to do. He told his superiors that, if he agreed to the role, he would have a mobile internet device so they could broadcast updates from inside the chimney. ‘I would then be able to take pictures of the event and myself wearing a gas mask and send them around the world,’ Kennedy reported. ‘He said I would be broadcast around the world and jokingly said I would be a hero. He said if we pulled it off it will be amazing.’

Kennedy may have wanted to take the lead climbing role in the protest, but he was informed by his supervisor that this was out of the question. Detective inspector David Hutcheson had been Kennedy’s cover officer for the last six years. He had been his shadow, tracking his movements, reading his emails and text messages, staying in nearby hotels in case of an emergency and sanctioning his every move. In theory, it was Hutcheson’s job to make sure Kennedy never crossed the line. On April 9, the eve of the planned Easter weekend protest, Hutcheson met with Kennedy. The two police officers went through a familiar ritual that takes place before covert operations: the senior officer reads aloud a set of guidelines called ‘instructions to undercover officers’, recapping what they have been tasked to do and the limits of their powers.

Technically, like all the other SDS and NPOIU officers, Kennedy was supposed to only ever be an observer, taking part in criminal activity as a last resort. His instructions for the next 48 hours were explicit: his job was to obtain ‘pre-emptive intelligence’ and explore ‘possible opportunities to disrupt’ the protest at the power station. He only had permission to hire a van and drive activists to the gates of the power station. He was prohibited from entering the site and barred from taking part in the demonstration.

That much was standard practice. But this particular operation would differ from the others in one crucial way. Rather than just monitoring the activists, Kennedy’s supervisors wanted him to gather evidence against them, using a specially modified covert watch.

The following morning he went to the Nottingham offices of All Truck Vehicle rentals and hired a 7.5-tonne lorry. He gave the manager of the company Mark Stone’s fake passport and driving licence, which contained a recent speeding fine. He used Mark Stone’s Maestro debit card to pay £278.48 for the rental and left a £500 cash deposit. The manager of the hire company later provided this description of the customer: ‘I would describe Mark Stone as a white male, 5ft 10in to 6ft tall, mid- to late 30s in age, of a medium build, with long brown hair tied in a ponytail. He had distinctive boss eyes, in that his eyes looked in different directions.’

Kennedy drove the van to the Iona primary school in the suburbs of Nottingham, where activists from across the country were scheduled to meet the following day. One campaigner had keys to a disused wing of the school and it was empty over the Easter break. If anyone asked, the activists planned to say that they were hosting a weekend of workshops, teaching sustainability through street theatre. To add credibility to this cover story, they had even advertised the Ecological Showstoppers weekend and printed hundreds of flyers for the fake event. All of the protesters who were heading to Nottingham knew, of course, that they would be involved in something more exciting than amateur dramatics. But they had no idea exactly what kind of protest was planned. Details of the protest had only been shared on a need-to-know basis – the core group of five had expanded now to around a dozen. Only they knew the magnitude of what was being planned.

Over the next 24 hours, scores of minivans would depart towns and cities across the UK and bring activists to Nottingham. Each minibus driver had a set of instructions, maps and a mobile phone with an unregistered SIM card. They were told to stay in touch with base at the Iona school for further directions. There was always the chance the protest would be pulled at the last minute, so the drivers were told not to set off on their journey until the phone beeped with a text message confirming the all-clear. If organisers had counted correctly, more than 100 activists would start arriving at the school the following morning.

Once everyone was at the school, they would be split into groups and given a detailed briefing about what the direct action involved and the specific role each individual would have if they decided to take part. It was inevitable that some people would drop out. This was a radical direct action protest, and anyone inside the power station was likely to face arrest for trespass. It was a risk some would take out of conviction to the cause, but not everyone. Those activists who agreed to the plan would be ferried to the power station in the middle of the night in three 7.5-tonne trucks, two people carriers and a Ford transit. The vehicles had already been packed with bolt cutters, angle-grinders, ropes, metal fences, food, concrete blocks and bicycle D-locks. They had prepared for every eventuality, except perhaps the possibility that they had already been rumbled.

The activists first began to grow concerned that police knew about their plan that evening. There were still only around a dozen activists at the school and some of them decided to take a quick drive around the power station to go over the route they planned to take the following night. They returned to the school with some bad news: parked around the power station, in what looked like strategic spots, were three police cars. ‘We just thought: Oh, no! Oh fuck! They know about it!’ says Tom. ‘It was just completely gutting. All of the work we had put in over the months and now it seemed there were police there guarding the station.’

It would have been foolish to attempt to break into the plant when the entrance was being protected by police. For a brief period, they considered changing the target of the protests, heading instead to Kingsnorth power station in Kent. After lengthy discussions, they decided it was impractical to switch power stations at the last minute. If the police cars remained outside the Nottinghamshire power plant, there was no choice – they would have to abandon the protest altogether. The organisers phoned the minibus drivers and told them not to set off in the morning; the protest would have to be put on hold until further notice. Tom remembers the look on the face of a stalwart of the protest scene, someone who, like him, had spent months preparing for this moment. ‘He looked absolutely broken. We all did, I suppose.’

There was a sombre mood at the Iona school the next morning. Activists had intermittently driven past the power station through the night and each time the three police cars were still there. It looked like they were on the verge of having to pull their plans. It was 8.30am on that Sunday morning when Kennedy told activists he would make one last visit to the power station to see whether police were still guarding the facility. He left the school in a taxi. Away from the activists, he would have been expected to call his superiors at the NPOIU, and explain that the protest was about to be called off because activists had seen police guarding the station. At that point, police could have chosen to arrest the dozen depressed-looking activists sat around the school. They could be sure they were arresting some of the key organisers, before all the other protesters had even started travelling to Nottingham. Instead, they chose a different course of action.

When Kennedy returned to the school 90 minutes later he was smiling. ‘They’ve gone,’ he told the other activists. ‘There are no police there. No cars. No nothing.’

The other campaigners were ecstatic. With police apparently no longer outside the power station, there was still a chance they could get inside. They sent messages to the minivan drivers: the protest was on again, and they should bring the activists to Nottingham as soon as possible. Tom recalls the irony of that decision: ‘We did not know we were phoning up activists across the country and telling them to come to Nottingham for what was essentially a police trap.’

Kennedy walked away to find a quiet corner of the room where he could not be heard. He rolled back his sleeve and lifted his watch to his mouth. ‘I am an authorised undercover police officer engaged on Operation Pegasus,’ he whispered. ‘This weekend, Easter weekend, I am together with a group of activists that are planning to disrupt Ratcliffe-on-Soar power station.’ He paused. ‘Shortly gonna go, to record briefings that subsequently take place throughout the day … The time on the watch is 10.06am.’

Kennedy’s reports to the NPOIU record what happened next. ‘People arrived at the school throughout the day. I recognised many people and knew them from various parts of the UK, including London, Bristol, Manchester, Sheffield, Leeds, Nottingham and Scotland.’ What Kennedy omitted from this report was that these people were by now his friends; some were pals from the Sumac Centre, including Logan.

The school was transformed into a buzz of activity by the arrival of the activists. Small parties arrived throughout the afternoon, unloading sleeping bags and carrying stocks of food into the school, while others slipped out of the back, to burn bundles of incriminating paper at a nearby allotment.

By the evening, all 114 activists were gathered in one room in the Iona school to hear a briefing about the planned protest. It was around that time that the pot-bellied police chief was briefing the riot squad in a warehouse a few miles away, preparing them for the impending mass arrests. In just a few hours, these two worlds would collide.

Kennedy was sat with his fellow activists in the school and, for the second time, activated his watch to work as a recording device. It picked up the excited murmur of more than a hundred campaigners against global warming packed into a single room. It was now dark outside and everyone who needed to be at the school had arrived. Above the chatter, the watch strapped to Kennedy’s wrist captured snippets of his brief discussion with a man sat next to him.

‘Gonna get a bit hot in here as well, ain’t it?’ Kennedy said.

‘Yeah, in about an hour’s time,’ the man replied.

‘Yeah.’

‘There’s no air flow and there’s lots of people.’

The conversation was interrupted by the sound of applause. Someone at the front of the room had stood up and was beginning to talk in a deep voice. It was one of the five organisers.

‘Everyone bear with us a bit. It has been a long night and this is amazing,’ he said. ‘The basic plan – there is a big power station over there, it emits 10 million tonnes of CO2 every year. That’s more than the 41 lowest-emitting countries in the world. We’re gonna go and shut them down for seven days.’ The man’s voice was drowned out by the sound of cheering and clapping.

Taking turns, the five activists explained various aspects of the plan. They stressed that even though people had travelled for miles to reach Nottingham, they should not feel obliged to participate in the protest. Each person had to decide if they wanted to be involved and agree to abide with strict safety procedures, avoiding interference with the water flow system and the potentially toxic cooling towers.

The direct action was laid out with clockwork precision. At precisely 3am, they would climb into their allotted vehicle and head to the power station in teams. The convoy of vehicles would first drive to a 24-hour café for truck drivers that was perhaps a mile from the power station. They would wait a few minutes and then drive off in a pre-planned order. Activists were divided into colour-coded groups, each assigned a different role. At the front of the convoy would be the people carrier and transit van containing the orange team. Their job was to force their way through the gate and ensure the other vehicles could get in. Once everyone was safely inside the grounds, they would use arm locks made out of fire-extinguisher shells to form a human barricade across the entrance, preventing police from accessing the site and buying time for everyone else.

The bronze team, dressed in suits, would make a beeline for the power station control room and calmly inform managers that a peaceful protest was underway, demanding they turn off the furnaces for the safety of those involved. Meanwhile, the blue team would head toward the nearby river – used to channel water into the cooling towers – and lock themselves to the pump facility. They were instructed not to tamper with the water pump mechanism – their role was simply to distract police and security guards by causing a nuisance. The silver team was also a decoy. Tasked with creating the ‘illusion of chaos’ inside the power station, they would splinter into small groups and lock themselves to any components they could find. When police arrived they would be baffled by the mayhem and forced to attend to activists locked on to various parts of the plant.

All of which was the perfect distraction, allowing the two most important teams in the protest to get on with their work. One was the black team, which would be driven to the power station in the back of Kennedy’s truck. Its role was crucial: immobilising the enormous conveyor belt that delivers coal into the four furnaces. It was a tricky climbing task. When Kennedy parked his truck inside the grounds of the power station, the back doors would be flung open and activists would come streaming out wearing harnesses and helmets and carrying rope equipment. Halting the conveyor system by pressing an emergency button, they would clamber on top and wrap their ropes around the huge, car-sized teeth used to scoop up the coal. Once secure, they planned to suspend themselves underneath the conveyor belt, preventing the power station managers from turning it on. Police would arrive to find a handful of protesters dangling 20ft off the ground and the furnaces would be starved of fuel and turned off.

If the protest went to plan, the ground would be prepared for the most important group of all: the green team of specialist climbers who by then would have scaled one of the four towering chimney stacks. High above the rest of the power station, they would be able to see the glow of Nottingham beyond distant hills, and a snake of cars making its way along the nearby M1 motorway. These activists would harness themselves to the stack to avoid being blown over in the wind and then begin the delicate procedure of entering the chimney, abseiling down and suspending their bat tent. By the time the sun appeared over the hills, they would spot the first TV satellite vans parked down below, as news spread of the occupation. Whoever was inside the tent would turn on a small laptop and begin broadcasting live around the world.

The bubble was popped shortly after midnight, when most activists were inside their sleeping bags in the school, trying to get to sleep. ‘Bang,’ recalls Tom. ‘Something gets smashed. Then we just see police storming in from every direction.’ A police video camera captured the scene inside one room in the school minutes after the raid: two dozen sullen-looking activists, staring at the floor in silence. The camera panned round at the sound of banging and glass breaking elsewhere in the building.

‘Sir, there is a locked door here,’ an officer shouted.

‘We’re going to enter here by force, OK,’ said another. ‘We haven’t got a key.’

Inside, some activists were hurriedly trying to eat scraps of paper containing their notes. Moments later, a senior police officer stood in the middle of the room and said: ‘You are all under arrest on suspicion of being involved in a conspiracy to commit aggravated trespass and/or criminal damage at a power industry facility.’ One activist began singing Vera Lynn’s ‘We’ll Meet Again’ and, slowly, everyone joined in. Then they began a chorus of Dolly Parton’s ‘9 to 5 (What a Way to Make a Living)’, to mock the banality of police work. ‘It was a surreal moment,’ one activist remembers. ‘But it lifted everyone’s spirits.’

The police raid was unprecedented. Throughout the history of the SDS and NPOIU, police always waited for a protest to happen before arresting the suspects. Sometimes there would be clever ploys to disrupt a planned demonstration. But never before had riot police stormed in to arrest this number of activists before they had even begun. This was the birth of a new strategy: wait for activists to gather in one place and then pre-emptively arrest them on charges of ‘conspiracy’. Kennedy had just facilitated the largest pre-emptive arrest – for any type of crime – in modern policing history.

Over the next few hours, activists were frogmarched, one by one, into the school gym where they were searched and made to pose for a police camera. Among the less co-operative was Kennedy, dressed head to toe in black and still wearing his watch. The arresting officers had no idea he was a police spy.

‘You’re being mass-arrested mate, just face here,’ a policeman told him. ‘You’ve been arrested and now you will be individually arrested by this officer.’

Kennedy turned to one side and stared into the middle distance. He looked drained. He was handcuffed and taken with the others to the waiting police vans parked outside the school to be transferred to a police cell. Among the last to be escorted out was Penny. ‘I remember walking out the school and there were so many flashing police vans,’ she says. ‘I couldn’t see anything but blue.’

The barn in Herefordshire was packed to the rafters with a drunken crowd. A bunch of veteran activists were turning 40 and this was their joint party: a weekend celebration in honour of eight old-timers who had been born in a year synonymous with protest: 1969. They had formed a rock band for the occasion called ‘The 69ers’, and the amateur musicians were about to take to the stage. They were all wearing black T-shirts printed with an image of a couple going down on each other in the so-called ‘69’ position. It was five months after the raid on the school, and many of the people in the barn were on bail over the planned occupation of the power station. Now, though, they were in a mood for partying. Logan looked the part on drums, his purple hair draped over his shoulders. So too did Kennedy, who was wearing a black trilby hat and posing with an electric guitar.

Kennedy had been waiting for this moment for months. It was the peak of his six-year deployment, a final moment in the spotlight for Mark Stone, his alter ego. Kennedy was the person who first suggested the joint 40th birthday bash for him and his friends. A trail of email correspondence reveals quite how much the party meant to the undercover police officer. One of the first emails came from Logan: ‘Following a suggestion from the venerable Flash Mark, over the last year or so we’ve made noises about having a joint party in 2009 for all those of us turning 40,’ he wrote. He summarised the idea of the weekend party, a kind of Earth First without the politics. To prevent ‘a ruck with the cops’ they would hire a venue instead of squatting in a field. It would be a family affair, with space for camping and activities for guests with children. Logan wanted to incorporate just a bit of political activity as ‘a great affirmation of our common politics’.

Kennedy could not conceal his excitement. He promised to DJ a ‘flash drum and bass set’ and even offered to pay for a Croatian band called Analena to fly over to perform a set. ‘Eastern Europe anarchist punk,’ Kennedy told his friends. ‘It keeps you young, a must-have for any 40th.’ He didn’t like Logan’s suggestion of adding politics to the mix, but told the others they could ‘crack on if you feel the need to wave banners in front of something’. Transport Mark promised to ‘rustle up’ a marquee from his Activist Tat Collective and said he would take care of logistics for the weekend. ‘Whatever we want,’ he told his friends. ‘It’s all possible.’

The police spy promised to write some poems for the occasion and said he would buy everyone themed printed T-shirts, joking he could get them from an ethical supplier who would guarantee ‘water, food, a bike and education for a family of 16 in Mozambique’. The celebrators calculated they could fit around 350 of their closest friends on the farm. Kennedy’s list of potential guests was testament to the friendships he had forged during his years undercover; it was filled with the names of hundreds of activists, including some from Germany. A flyer produced for the party was composed of pictures of John Lennon and Yoko Ono and a hippy confronting police with a flower.

In the dozens of emails Kennedy sent to his friends in the lead-up to the party, there was one topic he mentioned more than any other: the 69ers band. When his friends agreed to form the rock group ‘for one night only’, Kennedy replied: ‘Excellent, excellent. I have a lead guitar and an amp, I’m sure we can borrow drums from the bands that play. I also have a banjo and a large bongo. Or so I am told!’ He added: ‘I have lyrics already forming in my head for a bit of a song about all of us and 69 and crazy times.’ A few months later he told his friends he had been practising guitar ‘until my fingers bleed’ and learned ‘a bunch of tunes I can thrash out in an angsty punk style’. He seemed obsessed.

Of course, thoughts of his imminent debut on stage as a rock artist distracted Kennedy from darker thoughts. Dozens of his closest friends were on bail over the planned occupation of Ratcliffe-on-Soar power station. As a result of his betrayal, they were facing the prospect of jail. ‘I just felt so bad, I felt fucking awful,’ Kennedy reflected months later. ‘I did my job very well but I had got to a point in my deployment where it was becoming very hard to do those things against people who really meant a lot to me on a personal level.’ At the back of Kennedy’s mind, he knew that he may have to help prosecute his comrades. If that were to happen, it would be a significant departure for the NPOIU which, like the SDS, always avoided its officers appearing in court. ‘I’d be facing people I’d known for seven years – and who were really good friends – across a witness box,’ he said.

Even if the case against the activists never reached court, Kennedy was increasingly paranoid that activists would work out he was the mole in the Ratcliffe protest. Out of all the 114 activists who were arrested that night, Kennedy was the only one who was not represented by the same firm of London-based lawyers, Bindmans. Kennedy was furious with the NPOIU, who did not want to risk one of their operatives being represented by the human rights firm. ‘I said: look, everybody else has got a solicitor, Mark Stone hasn’t – it looks really odd,’ he said.

If Kennedy was compromised by being the only activist without a lawyer, the situation was made even worse when prosecutors began deciding who out of the 114 should be charged. A decision was taken to let most of the activists walk free, to concentrate on bringing charges against a smaller group for conspiracy to commit trespass. In a terrible error, Kennedy was placed in the smaller group who potentially had to go to court. When police realised the mistake, the case against Kennedy was suddenly dropped. He believed that made him stand out even more. Kennedy felt suspicious eyes were turning toward him. It was a concern that may have been shared by his supervisors at the NPOIU, who seem to have believed their prized agent was running out of time.

The truth is that Kennedy was not quite as exposed as he feared. Everyone knew Mark Stone as an eccentric character with a dodgy past, and he had a good excuse not to want to share the same lawyer as everyone else. Other people were suspected as possible infiltrators, but not him. When the birthday festivities got underway on the farm in Herefordshire on a warm September Friday, Kennedy must have felt reassured that his friends still trusted him. It was an impressive turnout for a 40th celebration.

Although Kennedy had not wanted a political festival, it was impossible to escape the fact this was a party for veterans of the radical protest movement. The menu consisted of black-eyed bean, nut and parsley stew with rice and green salad, and food was supplied by Veggies and the Anarchist Teapot, two collectives which had been feeding protest camps for years.

During the day, friends lounged on bales of hay and drank organic local cider in the sunshine. There were egg and spoon races, football matches, a mud-wrestling contest, a traditional Gaelic céilidh dance, cabaret and a barbecue. Inside one of the barns, Logan had pinned up photographs of all of the 40-year-olds, showing how much they had changed over the years. It was the first time that most of Kennedy’s friends had seen images of his previous life. One showed Kennedy as a young boy in the 1970s, stood next to his brother. Another, from the 1990s, captured a dazed-looking Kennedy at the end of a long-distance running race, caked in mud and wrapped in a blanket. His hair was bleached blond, separated by a centre parting and held back with a headband. Finally, there was a photograph of Kennedy a few years older, around 2002. It was the old Kennedy, the man as he was just before he went undercover in Nottingham. He was wearing the same dark sunglasses that helped to conceal his injured eye, but his hair was shaved short and his arms looked naked without the tattoos.

Another barn was converted into a music venue for the highlight of the weekend: the live Saturday-night performance by Kennedy and the 69ers. This was the part of the weekend that Kennedy had been looking forward to most. ‘He had been saying how it was his dream since he was a kid to be in a band and he had never had the chance,’ one friend says. ‘It seemed really genuine.’ Kennedy’s emails confirm his emotional investment in the band. He told friends he had purchased a DVD with ‘rare amazing event footage’ from 1969, to be projected on the wall behind them when they played. He hired a sound studio in Leeds so the band could rehearse more than a dozen times. And he helped plan a detailed set-list.

Everyone turning 40 had the chance to sing at least one song. The opening track would be the Stooges’ cover, ‘1969’, followed by ‘Anarchy in the UK’ by the Sex Pistols and T. Rex’s ‘20th Century Boy’. Logan would sing ‘White Riot’, the classic punk anthem by the Clash. Megan would also have a chance to come on stage to sing a song. Kennedy was determined that he should sing Johnny Cash’s ‘Folsom Prison Blues’, in honour of all the comrades who had gone to prison for the cause.

Kennedy emailed each of his friends in the band, giving them specific instructions. Clearly, he had given a lot of thought to the performance. Before each song, band members should take to the microphone and engage in ‘a little banter’ with the crowd. ‘Let’s not ramble on,’ he said. ‘I like to think it’s more about the punk rock.’ Before anyone walked on stage, they would play a pre-recorded tape, mixing a drum roll by American rock band MC5 and Jimi Hendrix’s ‘Star Spangled Banner’ medley from Woodstock in 1969. Via email, Kennedy told his friends that, after individualised introductions from the master of ceremonies, each band member would take it in turn to jump up onto the stage ‘to raucous applause, wolf whistling and screaming groupies’.

That was pretty much what happened on the night. The room was packed full of friends. The mix tape was played, building up to a crescendo before the band jumped on stage, smiling and waving. Megan did a turn on the microphone as a guest vocalist. Then the master of ceremonies, who was dressed in a tailcoat peppered with glittery sequins, took to the microphone and introduced Kennedy. ‘On guitar, direct from the bar!’ he shouted, as the crowd began to cheer. ‘Diamond geezer, he’s a rock and roll star! It’s Flash!’

The crowd went wild, but Kennedy was not feeling the love. He looked dejected for a reason. Moments before he was called onstage, he had received a text message from the NPOIU. ‘The operation is over,’ it said. ‘At least you had a great party and now it’s over.’ He had three weeks to ‘get out’ of Nottingham.

Friends recall a forlorn-looking Kennedy walking onto the stage in a daze and singing his Johnny Cash song. The band played the chorus again and again, accelerating the rhythm each time, as the crowd stomped and cheered. When the performance was over, Kennedy removed his guitar and walked through the barn, friends patting him on the back. Later, when the live music was over, he took to the decks to play one of his favourite records: KRS-One’s rap anthem: ‘Sound of da Police’.

Kennedy did not appear to want to talk to anyone. Friends presumed he must have been overwhelmed with the love and attention he was given. He told Logan: ‘I’ve never had so many people do something that is about me.’ It seemed the occasion had got to Kennedy and he was feeling melancholy. ‘Having a couple of hundred people love you and sing happy birthday to you – maybe that does unsettle you,’ Logan says. ‘But there was something not right in the way that he said it.’ Kennedy went to bed early, and the party carried on in his absence until dawn. ‘It was absolutely the best party I’ve ever been to, or expect to go to,’ says Logan. ‘Except now it has all been spoiled.’

The next morning, when hungover revellers awoke for Sunday brunch, Kennedy was missing. His friends found him lying in bed in the campervan he was sharing with Megan. He refused to come out. Word spread that he looked terrified and was hyperventilating. ‘He was freaking the fuck out,’ says Logan. ‘Panic attacks had never happened like that to Mark before. And after that, he was a very different person. He appeared jumpy, very paranoid.’

The next three weeks seemed to pass in a blur. Kennedy became frosty and cold toward his friends. He told them he was worried police were on to him and they might find the secret stash of money he earned as a drug courier. Once or twice, he phoned friends telling them in a paranoid, hurried voice that he thought he was being followed by plain-clothes detectives. He told his closest friends he wanted to start a new life and talked about moving to the United States to live with his brother, Ian. Just as generations of SDS and NPOIU officers had done before him, Kennedy was feigning some kind of breakdown, preparing the ground for his exit. However, in his case, the symptoms may have been genuine.

‘It seemed a bit fucking weird,’ admits Logan. ‘But the jumpiness and the paranoia was quite intense. He moved out of his house in Nottingham not long after. Well, he didn’t actually move out, he just didn’t come back. He sent a bunch of us in to clear the house for him. He said he was going put some stuff in storage, but basically we should get rid of the rest – the crockery, the furniture – just throw them out.’

Kennedy disappeared from Nottingham in October 2009 as abruptly as he had arrived six years earlier. His friends cleaned his house in his absence. He had stopped calling or answering emails, although he continued to speak with Megan on an almost daily basis.

That month, Kennedy was summoned by his managers to a meeting at an anonymous truck stop. It would have made a strange sight: a ragged-looking man with long hair stood by the roadside in sombre conversation with men in suits. Mark Stone was being dismantled. As traffic rushed past, the police spy formerly known as UCO 133 handed over his fake credit cards, car keys, passport and driving licence.

‘As of then,’ Kennedy says. ‘I’d vanished.’