A man whose work was synonymous with espionage, John le Carré was a world-renowned author of thrillers such as The Spy Who Came In from the Cold and Tinker Tailor Soldier Spy, but he was also an MI5 officer. In this programme, he talks to Nigel Williams about his involvement in covert activities in the interests of national security, reflecting on the morally questionable actions agents sometimes have to perform