The double agent, the ‘mole in the circus’ who pretends to be working for the intelligence service of his or her country, while secretly working for its enemies, is a central character in spy novels and their cinematic and television off-shoots. In the British case, this theme is not pure fiction. Between 1939 and the early 1950s Britain’s Special Intelligence Service (SIS, or MI6) was penetrated by a subsequently notorious group of double agents, known as the ‘Cambridge Five’. As students or later, they were recruited by Soviet intelligence in the 1930s, only to gain crucial postings in its British equivalent, or in the diplomatic corps, during the second world war. As such, they succeeded in sabotaging a large part of post-war Britain’s attempted resistance to Stalin’s dictatorship, and were not unmasked until the 1950s and ‘sixties. Chief among the ‘Five’, Kim1 Philby was the son of the noted Arabist H. St. John Philby. Converted to communism while a student at Trinity College Cambridge, he had been recruited by Soviet espionage in 1934.2 There is a whole library of books on his subsequent career,3 which can be briefly recounted. In 1940 Philby joined MI6, being first involved in preparing clandestine attacks on enemy targets, and counter-espionage. By the end of the second world war, he was the head of Section 9 of MI6, dealing with anti-Soviet operations, with the scandalous result that the section which was supposed to prevent Soviet espionage was itself headed by a Soviet agent.