Every now and again the National Archive at Kew gives us a gem, a rare glimpse into our nation's recent past and today promises to be one of those days.
The diaries of Guy Liddell are being declassified. Many of us won't have ever heard of him, but as Deputy Director General of MI5 from 1946 to 1953, he was dealing with security at the start of the Cold War.
The interesting thing about his diaries is the contact he had with the notorious Cambridge 5 spy ring, who were secret agents for the Russians - Kim Philby, Guy Burgess, Anthony Blunt, Donald Maclean and an other whose identity has never been confirmed.
The five are thought to have been recruited from Cambridge where thay were all undergraduates in the 1930s.
The most gripping entries though chart the months leading up to Burgess and Maclean's defections and the suspicions surrounding why they did it.
In early 1951 Liddell notes that for some years the Security Service had been looking for someone in Foreign Office circles, who had been leaking information to the Russians and that now “considerable suspicion rested on Donald Maclean”.
It's believed Philby learnt of this MI5 investigation and as the intelligence service prepared to interrogate Maclean, Philby sent Burgess to tell him he was in danger.
Maclean then disappeared followed by Burgess. In reality both men had defected to Moscow. In his diary, Liddell writes it's “pretty clear that the pair had gone off”.
Liddell records his conversations with Anthony Blunt following the disappearances, with both men believing it was unlikely that Burgess would have “sold himself to the Russians”.
Liddell also states he does not believe Burgess could have been a spy.
The diary entries refer to Burgess and Philby's friendship and then progess onto suspicions of whether Philby could be involved and plans to interrogate him.
Liddell recorded in December 1951 that the MI5 interrogator who spoke to Philby was "firmly of the opinion that he is or has been a Russian agent, and that he was responsible for the leakage about Maclean and Burgess".
However in his diary Liddell said he was not convinced Philby was working for the Soviets.
What seems clear is that Liddell was unwittingly confiding in members of the spy ring about their fellow members and in the process giving away vital information.
Later diary entries from 1952 chart the aftermath of the Burgess and Maclean disappearances and the growing suspicions surrounding Philby.
They also show MI5's shock at what had happened and the extent to which thay had been duped by the Cambridge Five.
Philby later defected to Moscow in 1963. As for Blunt, he confessed in 1964 and was given immunity from prosecution.
The diaries are extensive and by no means only focus on the Soviet spies, there are lots of other eyebrow-raising entries including Liddell's unfavourable impressions of the then Prime Minister, Clement Attlee, who he found to be "taciturn".
He also talks about secret Russian spy signals such as wearing a rubber band on your little finger and the possibility of MI5 making secret documents radioactive so alarms would sound if they were taken out of the building.