Tinker Tailor is loosely based on the real-life story of Kim Philby, who rose high in the ranks of British Secret Service before being caught in 1963 after 30 years as a Soviet spy. Philby was one of five spies whom the Soviets recruited while they were at Cambridge in the 1930s ("The Cambridge Five"). He was the best of the bunch. He could dissemble with ease. He was smart, funny, and charming, all of which contributed to his success. If he hadn't been caught, he may well have become head of British Secret Service, which would've been his chef d'oeuvre. He was (probably) allowed to escape to Moscow rather than holding an embarrassing trial in the UK (embarrassing for the UK government, that is).
Kilby had numerous friends in MI6, but none was probably closer than Nicholas Elliot, a fine spy in his own right, and the one who eventually confronted and extracted Kilby's confession in 1963. They and their families spent countless hours together, often vacationing with one another, and when Philby first came under suspicion because of his ties to two of the Cambridge Five who defected to Russia in 1951, Elliot was Philby's staunchest defender. Their friendship, however, did not prevent Philby from betraying his good friend's confidences by routinely passing on information to the Soviet Union that Elliot shared with him, information that sometimes led to the death of others.