What was it like to be a spy during the Troubles in Northern Ireland? HARDtalk's Stephen Sackur speaks to former MI5 agent Willie Carlin. He became an undercover spy within Sinn Féin, the political wing of the IRA, during the so-called 'dirty war'. He was dramatically extracted after his cover was blown. Now he’s written a book - Thatcher's Spy - about his experiences.