The seasons were on the turn; Londoners were reaching for their jumpers beneath a cobalt blue sky. A little before lunchtime on September 19 2011, a debonair academic was preparing to give a scholarly talk on the fifth floor of a prestigious think-tank overlooking the River Thames. Around 50 people had come to hear the presentation at the International Institute for Strategic Studies, which styles itself as the world’s leading authority on political–military conflict. Nigel Inkster, a director of the institute, praised the academic’s ‘encouragingly youthful demeanour’ and paid tribute to his ‘long and distinguished career’ in the Special Branch.

Up to the podium stepped Dr Robert Lambert, a tall, composed man in his late 50s, wearing a well-tailored dark suit and tie. Lambert delivered his hour-long lecture, about progressive alternatives to combating the threat from al-Qaida terrorists, with the confidence of a practised public speaker. He spliced his thoughtful presentation with a few jokes, sending ripples of appreciative laughter through the audience, and waved his hand in the air as he expounded the arguments in his newly published book. This was a carefully written manuscript that charted selected parts of his 26-year career in Special Branch ‘countering threats of terrorism and political violence in Britain’, but made no mention of the darker periods of his past. There was nothing to puncture his image as a progressive academic, the recipient of an MBE for ‘services to policing’. When Lambert stepped off the podium, he contentedly absorbed the applause from the audience, just one of many that had been convened from Singapore to Aberystwyth as part of his book tour.

He might have felt he was reaching the pinnacle of his career. In fact, the opposite was true. Over the next few months Lambert’s reputation would be left in tatters.

Lambert began his career in the police in 1977, aged 25. He joined the Metropolitan police, following in the footsteps of his grandfather, Inspector Ernest Lambert, who spent a quarter of a century in the force. Within three years, Lambert was in the Special Branch and soon after recruited into the SDS, joining the ranks of Conrad Dixon’s secret unit. Time and again Lambert would prove himself a master in the art of deception, a risk-taker with flair.

His undercover persona was Mark ‘Bob’ Robinson, a disarming, intelligent radical with a taste for danger. In 1983 – the first year of his deployment – Lambert met Charlotte, a vulnerable 22-year-old woman at an animal rights demonstration outside Hackney town hall in east London. Around 100 campaigners were outside the 1930s Art Deco building to put pressure on the council to sign up to a charter pledging an end to animal cruelty.

Lambert wandered over and asked Charlotte why she was wearing a stewardess-style colourful uniform, scarf and hat. She replied that she had come straight from work. The second time they met was more memorable: a demonstration against foxhunting in the Essex countryside. Charlotte started to fall for the older man. Lambert began picking her up before protests and dropping her back at her flat in east London, where she lived alone. ‘He told me that he worked as a gardener in north London,’ she says. ‘He got involved in animal rights and made himself a useful member of the group by ferrying us around in his van.’

Looking back, Charlotte feels she was targeted by the man she knew as Bob Robinson. ‘He was always around,’ she says. ‘Wherever I turned he was there trying to make himself useful, trying to get my attention. I think he was about 12 years older than me. It now seems that he worked to build a relationship with me, which developed into an intimate friendship and which became sexual. I believed at the time that he shared my beliefs and principles.’ Lambert was Charlotte’s first serious boyfriend and before long she was besotted.

Lambert always gave the impression of being a committed political activist. He chided his girlfriend for not doing more to stop the abuse of animals and encouraged her to take a more radical stance. He said that lobbying MPs and town councils would never bring radical change, arguing that only revolution would end the oppression of animals and humans in a capitalist society. ‘He would tease me for not being committed enough,’ she said. ‘I was a vegetarian but he encouraged me to become a vegan and he got me to become more involved in “direct action”.’

The police spy liked to flirt with the radical end of protest – and gave the impression he would break the law to further his cause. At weekends he visited London markets to stop traders selling and slaughtering chickens on the street, or stood outside butchers harassing customers. A friend of Charlotte says Lambert’s persona was seductive: ‘It was really an aphrodisiac that you had someone who wanted to do everything with you, who kind of took the risks that he took, and so it was a real rollercoaster in her life.’ After a few months, Lambert and Charlotte appeared inseparable at demonstrations; friends commented they had an air of Bonnie and Clyde. Lambert was often protective of his younger partner too. He once rushed to help her when some foxhunters threw her into a lake.

Within a few months, the pair were an established couple among radical protesters in London. Rather than sitting in the back of the van, Charlotte was now in the front seat, beside the charismatic driver. ‘Although Bob had a bedsit, he would stay with me. We set up home together. He would sometimes go off for a short while saying he had to visit his dad with dementia in Cumbria and sometimes he went off saying that he had a gardening job. Most the time while we were together he lived with me.’ It was of course a double life. Lambert’s father did not have dementia and did not live in Cumbria. His periods away from Charlotte were instead spent living a more conventional life with his wife and children in suburban Herefordshire. For at least five days a week, however, Lambert was with Charlotte.

Many SDS officers struggled to maintain their double lives. Not Lambert. He appeared to relish his duplicity, switching effortlessly between two worlds that were poles apart. Lambert was an ambitious man. He knew that a successful deployment – one in which he accessed the furtive underground networks of animal rights campaigners – could propel him up the ladder at Special Branch, and Charlotte was a key part of his equation. One of the hardest challenges for covert officers is turning up out of the blue without friends or family to vouch for them. They arrive in their late 20s or early 30s with sometimes feeble excuses for their sudden interest in politics.

Acquiring a girlfriend was an easy way to fill the gap, making an undercover police officer seem like a real person. Having Charlotte by his side helped Lambert to douse any questions about who he really was. Although she was not the most radical campaigner, she was a recognisable face, and had been interested in animal rights issues for around two years. Lambert exploited the credibility his girlfriend had already earned. ‘One day, Bob wasn’t there,’ recalls a friend of the couple. ‘And then Bob was there. He was everywhere.’ There seemed no protest Lambert was not interested in. Unlike some of his more highly strung comrades, he had a relaxed charm about him.

‘Bob was not extreme in his language,’ says Martyn Lowe, a radical librarian. ‘He was calm, reasonable, smiling. In politics you get nutters who sort of rant and rave – he was the complete opposite to that.’ Paul Gravett, another activist, recalls Lambert as ‘an affable, nice, fun guy who knew a lot about things. He was not a cardboard activist, he had real depth to him.’ He remembers Lambert as ‘very convincing as a sort of alternative person’ who rejected the greedy individualism associated with Thatcherism, claiming instead to embrace a non-materialist lifestyle. He claimed to be into music – attending Glastonbury Festival and sharing Van Morrison albums with friends – and showed a keen interest in the left-wing literature stocked at Housmans, the radical bookshop near King’s Cross station.

Lambert was well versed in political theory, and, according to one of his friends from the time, was ‘pretty well organised intellectually’, able to debate the finer points of anarchist philosophy. The spy quickly developed the right patter. Adopting the doctrines of the animal rights movement, he condemned speciesism – the idea that humans have more moral rights than animals – and claimed he did not believe in keeping pets because he rejected the idea he should control an animal. In a letter he sent to a campaigner in 1986, Lambert wrote about a campaign to stop trains carrying nuclear waste through London. He called them ‘nukiller trains’ – the favoured term of derision among anti-nuclear campaigners. One another occasion, when scuffles broke out between protesters and police at a demonstration, Lambert immediately took out his camera to record what he called ‘police repression’.

Soon Lambert was throwing himself into several different strands of political activity, becoming involved in squatting, free festivals and anti-nuclear weapon camps. The mid-1980s were a tumultuous time in British politics as Margaret Thatcher sought to impose her will on the British people. Lambert and Charlotte were among those who resisted. ‘They were like warriors trying to fight against it all,’ recalls a friend of the couple. ‘It was a really unjust time, a really divisive time. You either made loads of money or you were living in real poverty.’ One of the big clashes took place in Wapping, east London, where the newspaper magnate Rupert Murdoch took on the print unions by sacking 6,000 workers. There were regular confrontations between police and supporters of the strike – more than 1,200 people were arrested. Lambert and Charlotte were often at the heart of the protests. The group Lambert seemed most interested in was a small, environmental group called London Greenpeace, which bore no relation to the larger campaign with the same name. London Greenpeace was a radical anti-capitalist group and Lambert quickly found his niche, speaking eloquently at meetings and writing the group’s propaganda.

Although he made a convincing activist, Lambert was never dour. His friends considered him a great drinking companion who liked to socialise at pubs like the Rising Sun in Euston Road. In the summer of 1986 he hosted a party at his flat on Talbot Road in Highgate when he moved out. He claimed to be earning a cash-in-hand income as a gardener in well-heeled properties in nearby Hampstead and elsewhere. A photograph from the era shows a lean, topless Lambert in white jeans and dirty trainers mowing a lawn. One day, to bolster his cover story, he hired two fellow activists to help him clear a garden in Surrey. He also told friends he had a side job driving mini-cabs, touting illegally for customers. True to his non-conformist lifestyle, he claimed he didn’t have a bank account and preferred working in the black economy to stay off the government’s radar and avoid paying tax.

Tracing the development of Lambert’s undercover deployment shows a master at work. He began on the fringes of radical protest, made friends, persuaded activists that he was useful, and then began the gradual process of working his way into the heart of the action. In the early part of his time in the field, Lambert just turned up at the kinds of publicly advertised demonstrations that rarely attracted more than a handful of uniformed police. One – a protest against food manufacturers Unigate – took place outside central London’s Dorchester Hotel in September 1984. Photographs show Lambert among a smattering of campaigners passing out handwritten leaflets and berating company shareholders en route to an annual meeting. He had even penned the leaflet that called on activists to make a stand against Unigate’s directors. ‘Let them know that there is no place for callous meat production in a caring society,’ the leaflet said. ‘Animals would prefer not to end up as Bowyers Pork Sausages.’ Lambert ended the leaflet with the popular animal rights slogan: ‘Meat is murder.’

However, from the outset Lambert was adept at cultivating the impression that his moderate exterior belied a more radical edge. ‘Bob gave off the impression he was doing a lot of direct action but one could never put one’s finger on it,’ says Lowe. ‘One always had the suspicion he was into something heavier. He never talked about it directly.’ Gravett remembers that soon after he met him, Lambert ‘let it be known’ that he dabbled in illegal protest. He said that Lambert even told him he had once dressed up as a jogger and poured paint-stripper over the car of a director of an animal-testing laboratory. Truth or fiction, the story helped Lambert portray himself as a secret firebrand. It was the persona he needed to fulfil what had become the main objective of his mission – penetrating the intensely furtive, hard-core wing of the animal rights movement: the Animal Liberation Front.

Since 1976, the ALF has carried out thousands of unlawful protests to stop what it perceived to be the abuse of animals. It has a reputation for extreme acts of sabotage that have caused millions of pounds of damage. Police view the group as an underground terrorist network. ALF activists have targeted laboratories, abattoirs, butchers’ shops, fast-food restaurants and factory farms. Their more radical tactics have included arson, break-ins, destruction of equipment, harassment of scientists, death threats and hoaxes. Campaigners in the ALF have been best known for the photographs in which they were disguised in black balaclavas and camouflage jackets triumphantly posing with rabbits or dogs they claimed to have ‘liberated’ from laboratories. For many years, the group was personified by its figurehead: the diminutive Ronnie Lee, a man widely recognised by his round glasses, beard and flat cap.

In the 1980s, the ALF was growing and its attacks were becoming more destructive. The ALF claimed it only attacked property and made sure that people were not harmed, but conceded its approach was combative. Keith Mann, an ALF activist jailed in 1994 for 14 years for sabotage, told the BBC: ‘One man’s terrorist is another man’s freedom fighter. Call us violent, call us terrorists, call us anarchists, they’re all used regularly. All we’re asking for is change. We want people to stop using violence against animals.’ The ALF was an obvious target for the SDS – one of the most important political groups the unit had to monitor. But the group presented a headache for senior officers.

ALF campaigners operated through a diffuse subterranean network of small activist cells. The ALF had no membership and no leaders. It encouraged the idea of autonomy; any activists could organise their own direct action and claim responsibility for it under the banner of the ALF. With no obvious structure, its followers worked in a fluid, loose way, coming together to execute a direct action and then quickly dispersing. Often attacks were carried out by solitary individuals. Each disparate cell waxed and waned according to the enthusiasm of its members and the ability of police to detect what they were up to. By 1985, police felt they were losing the battle.

A special squad was set up to catch ALF campaigners, who were subjected to telephone taps and extensive surveillance. Special Branch recruited an ever-expanding team of informants from across the animal rights movement in a desperate bid to find out what was going on. By the 1990s, police were running around 100 informants in animal rights groups, according to Ken Day, a former Special Branch man. He says some were being paid as much as £10,000 a year for intelligence on the ALF and its associates.

A confidential Special Branch document reveals that the SDS first deployed one of their operatives to the animal rights movement in April 1983. ‘At this time, the major related policing problem was public disorder at large-scale demonstrations. Hence it was a relatively easy matter for him to infiltrate … and obtain useful intelligence concerning public order events,’ the document notes. It says of their spy: ‘Essentially, he was working amongst well-meaning, idealistic campaigners … Within their ranks was a small but growing number of militant activists prepared to take the law into their own hands.’ Not long after that first deployment, the SDS deemed it necessary to send in a second operative and chose Lambert, one of their brightest young recruits. The document contains Lambert’s own assessment of the task ahead, in which ‘hard-won credibility and trust are prerequisites to high-grade intelligence’.

After first establishing himself among more moderate activists, Lambert set about befriending campaigners suspected of being in the ALF. One was called Geoff Sheppard. ‘In my eyes he was a totally credible activist,’ Sheppard says. ‘I felt no doubt whatsoever, you might say, that he was one of us.’ Sheppard recalls how Lambert drove protesters around the country to support fellow ALF activists who were being prosecuted. They travelled up to Sheffield when Ronnie Lee and others were put on trial for conspiracy to commit arson. Lee, a former trainee solicitor, was jailed for 10 years in January 1987. Sheppard, Lambert and others complained that the sentence was harsh, telling each other that the treatment of their comrade should spur them on to carry out further attacks.

Just as he had done with Charlotte, Lambert made Sheppard feel the pair had a special connection; they were locked together in the struggle. A photograph captures the two men together in Lambert’s van. The long-haired police spy is at the wheel of his battered vehicle, looking out the back toward his friend. ‘I believed in him, and I liked him, and I thought he was a friend of mine,’ says Sheppard. ‘I trusted in him 100 per cent.’ Both men were frequently seen at demonstrations and were involved in what Sheppard calls ‘the legal stuff’. But few knew that they were also committed ALF activists. According to Sheppard and Gravett, Lambert even produced a well-known ALF leaflet from the era which summed up the group’s philosophy under the stencilled headline: ‘You are the Animal Liberation Front.’ ‘You wouldn’t leave it to others if your friend was being beaten up in the street,’ it said. ‘In the same way you can’t stand by as thousands of animals are slaughtered every day.’

It was this uncompromising approach that led the ALF to wage a sustained campaign aimed at economically damaging companies that sold animal products and provided Lambert with an opportunity to make his name. The ALF was turning its attention to chains of department stores like House of Fraser, Debenhams and Allders, all of which sold fur. Over three years from 1984, activists had planted more than 40 incendiary devices to set fire to shops up and down the country. In one particular spree, there was a series of co-ordinated attacks on House of Fraser stores across northern England, from Altrincham in Cheshire through to Blackpool, Harrogate and Manchester.

The media called the arson attacks ‘firebombs’, but in reality the homemade contraptions, designed to cause tiny ignitions, were rather basic. Devices the size of cigarettes boxes were placed under flammable objects such as an armchair or settee in the stores. The ALF insists that the devices were designed to go off at night so that people were not harmed – typically, they would be laid in the afternoon and timed to ignite just after midnight. The aim was to cause enough of a fire to set off the sprinkler system, avoiding a full-scale blaze but flooding the store extensively and ruining the stock. The top floor was favoured as all the floors underneath would be drenched.

The ALF hoped the campaign would inspire copycat actions around the country. A step-by-step guide on how to make the devices was quietly circulated around cells of activists. Under the misleading rubric ‘Interviews with Animal Liberation Front activists’, the closely typed booklet contained intricate diagrams involving nails, batteries, nail varnish, tweezers, watches and washing-up liquid. The instructions could be mastered by a clever activist with enough time on their hands, while all the household ingredients were available in regular high-street shops.

Sheppard recalls the moment in 1987 that a trio of ALF activists concocted a plan to set fire to three branches of Debenhams in an attempt to force the department store to abandon its fur products. ‘Myself, and Bob, and one other person got together and formulated a plan,’ he said. According to another confidential source who knew about the plot, the components for the attacks on branches of Debenhams in and around London were assembled in an empty squatted property. After a period of experimentation, the trio believed they had improved the rudimentary design stipulated in the booklet.

Lambert was in a prime position for a covert agent. He confided to one friend he was ‘deeply involved’ in the ALF and spent hours defending the ethics of its hard-line tactics. Another friend recalls Lambert telling her that he was going to set fire to the department store. She says she ‘spent ages trying to persuade him not to, that it was a bad idea’. Lambert was unmoved: he portrayed himself as being willing to undergo a heroic sacrifice, risking his own freedom to prevent the sale of fur. The question was: how far would the undercover police officer go?

According to Sheppard, his friend Lambert was intimately involved in the conspiracy. Sheppard’s testimony about the attack on the Debenhams store – and the part he alleges Lambert played – was highlighted in a parliamentary speech by Caroline Lucas MP in June 2012. Lucas told the House that Sheppard had claimed that he, Lambert and a third activist were part of the plan to target three branches of Debenhams. The trio each collected two devices during the day on Saturday, July 11 and then headed off to their designated branch. One was in Luton, another in Romford. The third, which Sheppard said the police spy was going to target, was the Harrow branch in north-west London.

That night, Lucas reported, Sheppard says he returned from setting fire to his allotted store and turned on the radio to listen to the BBC World Service. The newsreader announced that three Debenhams stores had been subject to arson attacks – including the branch in Harrow. According to Lucas, Sheppard said in his testimony: ‘So obviously I straight away knew that Bob had carried out his part of the plan. There’s absolutely no doubt in my mind whatsoever that Bob Lambert placed the incendiary device at the Debenhams store in Harrow. I specifically remember him giving an explanation to me about how he had been able to place one of the devices in that store, but how he had not been able to place the second device.’

Lambert has consistently denied planting the incendiary device in the Harrow store. Whatever the truth, the simultaneous strike that night had by far the biggest impact of the ALF’s campaign for all the wrong reasons. Although police would have had advance notice of the plan, through Lambert, the sprinkler system at the Debenhams store in Luton’s Arndale shopping centre had been turned off to be repaired. There was nothing to douse the small fire ignited by one of the incendiary devices in the menswear department. Firefighters discovered the fire at 1.50am on Sunday. It took 80 of them, pumping water from 10 engines, until 6.39am to bring the blaze under control. Four floors of the store were gutted, and the store was shut for weeks. A black-and-white photograph from the morning after shows a fireman hosing down a mass of smouldering debris that looked like the scrunched aftermath of an earthquake. Insurers later calculated that the damage, including loss of trading, amounted to £6.7m at the Luton store alone – far more than any of the other arson attacks in the ALF’s campaign against the sale of fur by high street stores.

The store at Harrow was also badly hit, costing Debenhams £340,000. The fire alarm went off just before 1am, alerting the police and the fire brigade. Police constable Simon Reynolds from Wealdstone police station arrived within minutes and found ‘a thick blanket of smoke’. With the help of a torch, PC Reynolds and a colleague discovered the sprinklers working in ‘an area badly damaged by fire and water’ in the section of the store selling luggage on the first floor. The carpets, clothing and goods were sodden.

John Horne, of Scotland Yard’s anti-terrorist branch, confirmed that ‘an improvised incendiary device had functioned against the wall in a display area’. He told the court: ‘Amongst the debris I was able to identify the heavily charred remains of a small cardboard box, an alarm clock and what appeared to be the remains of a vehicle light bulb, and a dry cell battery.’

Police testified only one device was planted at the Harrow store. Two devices in the other Debenhams stores had failed to ignite and so police found them intact. Care had been taken to ensure no fingerprints were left on the small homemade devices, which had labels stuck to the side: ‘Warning – Do not touch, ring police, Animal Liberation Front.’ In Sheppard’s version of events, Lambert had again earned his spurs as an ALF activist during the attack on Debenhams. ‘I was already confident in him anyway but after that, I would have had absolutely no doubts whatsoever that he was a genuine ALF activist, because it simply would not have entered my mind that a police officer would carry out such an action.’

The attacks bore all of the characteristics of an ALF mission executed without a trace. If it were not for Lambert, detectives might never have worked out who planted the devices. Police made their arrests a couple of months later, bursting into the ground-floor bedsit of a corner house in Hillside Road in Tottenham in north London at 4.50pm on Wednesday September 9.

Sheppard and Andrew Clarke, the other ALF activist who would ultimately be convicted over the attacks on Debenhams, could not have been caught at a worse moment. They were in the middle of assembling a fresh round of incendiary devices for another wave of attacks. They were wearing rubber gloves, to keep their fingerprints off the devices, and sitting around a table in the tiny, spartan room. Spread out in front of them on pages of the Guardian newspaper was the paraphernalia for making four more devices – dismantled alarm clocks, copper wire, bulbs, batteries and nail varnish. The soldering iron being used by the pair to make the devices was still hot.

As Sheppard puts it: ‘There was a kick on the door at which point the door came swinging open very violently. At least four or five large suited men came through that door and we were caught red-handed.’ Lambert, however, was not with them. He seemed to have been the lucky member of the trio who had got away.

Months later, Sheppard and Clarke were sat side by side again, this time in the dock at the Old Bailey, listening as the prosecution laid out their case against them. ‘They were in the process of what was clearly a well-practised method of constructing incendiary devices similar in every significant respect to those used at Harrow, Luton and Romford,’ Victor Temple, for the prosecution, told the court. He explained how, after arresting Sheppard and Clarke, detectives seized a black woollen mask, ‘incriminating’ ALF literature, press cuttings of the previous attacks on Debenhams, and ‘lists of butchers, fur shops etc’.

Looking back, Sheppard says that the tip-off for the raid was so accurately timed that it ‘obviously came from Bob Lambert’. The police spy was one of the few people who knew that his friends Sheppard and Clarke would at that very moment be constructing more devices in the flat. But at the time, Sheppard never suspected Lambert. Indeed, for another quarter of a century, despite years languishing in jail trying to work out who the grass had been, Sheppard never once considered that his friend Bob Robinson had betrayed him.

In part, that can be explained by Lambert’s deceptive skill when his friend was put behind bars: feigning empathy for his comrade and supporting him at every step. In a letter to another activist in March 1988, Lambert explained how he was desperate to ensure that enough supporters were visiting Sheppard, who was on remand. ‘I just had a feeling that no one would be visiting him on the Sunday and I was getting very frustrated,’ he wrote.

Behind bars in Wandsworth jail, Sheppard felt grateful when Lambert visited him and gave him a gift – a book about philosophy. ‘I remember thinking, “Bob’s still there for me.” Actually he was the guy who put me there,’ says Sheppard. However, at the time, Sheppard and others had no reason to wonder why and how the third man in their cabal had got away. ‘For 24 years I have believed that my friend, what I thought was my friend Bob Robinson was on the run and had most likely gone to a different country and probably made a new life for himself,’ says Sheppard. ‘And I just thought – good for him, he was the lucky one that managed to get away.’

In June 1988, Sheppard and Clarke were convicted of the arson attacks on all three Debenhams stores, which caused £9m damage. Sheppard was jailed for four years and four months, and Clarke for three and a half years. Judge Neil Denison told the pair: ‘I am not going to spend time pointing out to you what in my view are the errors of your ways for the very good reason you wouldn’t pay attention to what I’d say.’ Sheppard and Clarke already had a record of breaking the law in protest against animal cruelty. At the time of their arrests over the Debenhams attacks, Clarke had a conviction for damaging property during an anti-fur trade demonstration, while Sheppard had breached a suspended sentence for smashing a butcher’s window. In the 1990s, Sheppard spent another four years in jail after he was caught with components to make incendiary devices and a shotgun. Both men say they no longer take part in illegal protests.

Sheppard and Clarke made an odd pair. Sheppard was an unemployed 30-year-old who professed an unwavering dedication to his cause, even if that meant breaking the law. Taciturn and straightforward, he says it as he sees it. Clarke, a gardener for a London council, was younger at 25, and, as he now admits, a touch naïve. He is often given to long, involved explanations of events. The pair would not naturally be friends, but any movement needs all sorts of people.

One interpretation of Lambert’s infiltration of the ALF is that it was a superb triumph for the police. The undercover operative had been buried deep enough in the group to secure intelligence that jailed two committed campaigners in the midst of an arson spree involving a well-known department store. And Lambert did it with aplomb, displaying a cunning that cemented his reputation as one of the finest spies ever to serve in the SDS. His escapades would be recounted for years to come, burnishing his credentials as one of the most committed ‘hairies’ ever deployed in the field. There is even grudging respect from Sheppard. ‘There is part of me that does feel betrayed by Bob because I genuinely felt that we were mates,’ he says. ‘But on the other hand, now I know that he was an undercover officer doing a job of work, I suppose you have got to, in that sense, hand it to him in a way. He was very clever at what he did.’

But would an officer in Lambert’s position who did carry out a criminal act cross a line? Broadly speaking, police chiefs can authorise an undercover officer to participate in criminal acts if they can show that it would help detect or prevent a more serious crime. They are not usually permitted to be involved in the planning, instigation or execution of crime. Nowadays covert policing is regulated by the Regulation of Investigatory Powers Act 2000, which requires advance authorisation from senior officers. But in Lambert’s time, the rules were more vague. The SDS considered the arrest of its spies, and their occasional participation in crime, a hazard of the job that could always be ironed out with a quiet word with someone senior at Special Branch.

A practice had emerged in which SDS officers who committed crimes quickly reported what they had done to a supervising officer. They were then provided with retrospective authorisations, recorded over the years in top-secret green files, known through their filing code: 588. It was not carte blanche approval to commit any crime, but it gave SDS spies licence to blur the lines when out in the field. Most of the time, the crimes they were committing involved trespass, breaches of the Public Order Act or minor acts of criminal damage.

The accusation levelled against Lambert is of a different order: that he encouraged and even participated in an arson campaign that caused millions of pounds of damage. Lambert has firmly denied that he planted the incendiary device at the Harrow store of Debenhams but takes credit for jailing Sheppard and Clarke. In a carefully worded statement, he says: ‘I was deployed as a Met Special Branch undercover officer in the 1980s to identify and prosecute members of the Animal Liberation Front who were then engaged in widespread incendiary and explosive device campaigns against vivisectors, the meat and fur trades. I succeeded in my task and that success included the arrest and imprisonment of Geoff Sheppard and Andrew Clarke.’ Denying the accusation over the arson attacks, he adds: ‘It was necessary to create the false impression that I was a committed animal rights extremist to gain intelligence so as to disrupt serious criminal conspiracies. However, I did not commit serious crime such as “planting an incendiary device at the Harrow store”.’

On one reading of the contrasting accounts, Lambert should be believed over Sheppard. It is the word of a long-serving, decorated police officer against that of a convicted animal rights campaigner with an obvious axe to grind.

Even if that is the case, there remain a host of difficult questions for Lambert and the SDS. If Lambert was involved in the arson campaign because it was deemed necessary to avert a more serious crime, who authorised his mission? Two former SDS officers say the unit would never have countenanced one of its spies taking part in sabotage of that severity as it would have been too risky and foolhardy.

And there were of course other options. Given they had advance warning of the plot against the three Debenhams stores, why did the SDS allow the arson attacks to go ahead? Why did they not intervene sooner, arresting Sheppard and Clarke as they were about to carry out their first wave of attacks?

Finally, there is the mystery of the fire in the Harrow branch of Debenhams. The luggage section of the department store was undoubtedly scorched by flames. If Lambert did not start the fire, then who did?