Anthony Blunt, aesthete, communist, homosexual, MI5 agent and Soviet mole, was Surveyor of the King's Pictures and Director of the Courtauld Institute. Betrayed in 1963, he voted for Margaret Thatcher in 1979. Late that year, she was to expose his treachery and strip him of his knighthood. While the other Cambridge spies (Philby, Burgess and Maclean) subordinated their lives and careers to espionage, Blunt had a separate passionate existence. His reputation as an art historian was second to none: he made an enormous contribution to the establishment of art history as an academic discipline;...