For two years we have delved into a particularly secretive part of the secret state. It is not possible, for obvious reasons, to acknowledge in public the many people who went out of their way to enable this story to be told. They know who they are. There were some, however, who did not help at all. Mark Kennedy did not co-operate with this book. On the occasions he or his family are quoted, the remarks are taken from published interviews with the Mail on Sunday, Guardian, BBC and Rolling Stone. The Metropolitan Police officially refused to co-operate and answered virtually none of our questions. We asked for an interview with a senior officer who could give the official view of these undercover operations, or assistance ‘on background’, which is almost always provided for projects on this scale. The force refused, saying undercover policing ‘is by definition a covert activity and it is imperative that we safeguard our working practices and certain aspects of our decision making, to protect the men and women who undertake this dangerous work.’

In the face of official obstruction, we relied on the courage of confidential sources. A number of ‘insiders’, past and present, have helped us, but we owe particular thanks to Pete Black. This book could never have been written without his bravery. Pete has not received any payment for this book. At his request, a portion of the royalties is being donated to a local branch of the mental health charity, Mind. We thank Tony Thompson for introducing us to Pete, who was at the time known as Officer A.

A number of people spoke on the condition of anonymity, either to protect their privacy or because they feared retribution from police. As a rule, all references to people by first names only (e.g. ‘Megan’) are pseudonyms. We are grateful to the many other activists who agreed to break with tradition to be named on the record.

Our book was made possible by senior executives at the Guardian who allowed us the time and freedom to investigate these stories and then turn them into a book. Particular thanks to Alan Rusbridger, Ian Katz, Dan Roberts, Dan Sabbagh, Jan Thompson, Sara Montgomery and Katie Roden. We would like to thank our main researcher, Sorcha Pollack, and three reporters who helped along the way: Rowenna Davis, who worked briefly at the Guardian, and Meirion Jones and Richard Watson, from BBC Newsnight. Thanks also to our friends at ITN Productions.

We’re grateful to three solicitors who represent the targets of the undercover operations – Mike Schwarz, Jules Carey and Harriet Wistrich – and two media lawyers who facilitated publication, Sean McTernan and Gill Phillips. We are also indebted to the careful skill of Lindsay Davies, who edited the manuscript, and Tom Lewis, whose advice early on helped shape it.

We have in places been greatly helped by journalists and researchers who have gone before us. In chapter two, we quoted from Peter Taylor’s BBC 2002 series on MI5 and the Special Branch, and his accompanying Guardian article of October 23 2002, which disclosed the existence of the Special Demonstration Squad. We also drew from a BBC Newsnight programme on May 28 2008 on the policing of the 1968 anti-Vietnam war protests. Solomon Hughes passed on documents he had uncovered in the National Archives in Kew, London, about those protests. We also quoted from a BBC Radio Four File on Four programme broadcast on October 2 2012 and, in chapter five, drew on two books: McLibel – Burger Culture on Trial by John Vidal (Macmillan, 1997) and Secret Manoeuvres in the Dark – Corporate and Police Spying on Activists by Eveline Lubbers (Pluto Press, 2012).

Authors regularly acknowledge the tolerance of their partners while they disappear for some time. We are no different. Rob would like to thank his wife Caroline for her forbearance and support. She may have got bored of him talking about undercover police, but she did not show it. Paul would like to thank Kay for encouraging him to pursue his obsessions. She was unwaveringly supportive, right from when the story first broke.

Finally, like all journalism, this book is merely a first draft of the history. We recognise we have only scratched the surface. If you can help us improve the record, then we would urge you to get in touch via Rob.Evans@guardian.co.uk or Paul.Lewis@ guardian.co.uk. We hope this book will be an incentive, in particular, to the South African resident, the Scottish hotelier, the pensioner in Lincolnshire and the second spy who learned the consequences of fathering a child undercover.