Early one morning in March 1985, as he climbed the six steps of Margaret Thatcher's prime-ministerial jet on the runway of RAF Aldergrove, little did Willie Carlin know the role Freddie Scappaticci played in saving his life.

So began the dramatic extraction of Margaret Thatcher's key undercover agent in Sinn Fein Willie Carlin, aka Agent 3007. For 11 years the former British soldier worked alongside former IRA commander Martin McGuinness in the republican movement's political wing in Derry. He was MI5's man at McGuinness' side and gave the British State unprecedented insight into the IRA leader's strategic thinking.

Carlin worked with McGuinness to develop Sinn Fein's election strategy after the 1981 hunger strike, and the MI5 and later FRU agent's reports on McGuinness, Adams and other republicans were read by the British Cabinet, including Margaret Thatcher herself.

When Carlin's cover was blown in mid-1985 thanks to one of his old MI5 handlers being jailed as a Soviet spy, Thatcher authorised the use of her jet to whisk him to safety. Incredibly, it was another British super-spy inside the IRA's secretive counter-intelligence unit, the nuttin squad, who saved Carlin's life. The Derry man is perhaps the only person alive thanks to the information provided by the jewel in the crown of British military intelligence Freddie Scappaticci, aka Stakeknife.

In Thatcher's Spy, the Cold War meets Northern Ireland's Dirty War in the remarkable real-life story of a deep under-cover British intelligence agent, a man now doomed forever to look over his shoulder.