An article in Britain's Sunday Times newspaper at this link: The Sunday Times and Kim Philby - Times Online publishes excerpts from the just released memoirs of former Sunday Times editor Harold Evans. It details the story of how that newspaper's investigative reporters discovered in 1967 that Kim Philby had been both a senior MI6 officer and a long-time spy for the Soviet Union, despite fierce institutional resistance to this line of inquiry from former and current members of MI6 and from the British Foriegn Office, paired curiously with repeated attempts to deceitfully minimize Philby's importance. This ultimately escalated to official demands under the Official Secrets Act not to publish the story, demands that were ignored.
A number of the reporters mentioned by name in this piece later went on to write important books about Kim Philby and other spies of the later half of the 20th century. They include Philip Knightly, who apparently extracted one of the key clues in this puzzle from a burned out and impoverished ex-MI6 officer by treating him to a liquid lunch of several brandies.
Embedded below is a 4 minute news clip from 1955 wherein an assured Kim Philby persuasively and effortlessly lies while repeatedly denying that there was any truth whatsoever to similar allegations that had been made against him (and then retracted under threat of a libel suit) more than a decade before this article was ultimately published in The Sunday Times in 1967.