The national press has been thrilling to revelations from a release of documents in the National Archives which detail the extent of espionage activity in Britain, and MI5’s sometimes ham-fisted attempts to counter it.

The story of the Cambridge Five has long attracted massive interest, with its overtones of an establishment very nearly brought down from within over the decades surrounding World War II. But it is worth examining all the sources to ascertain the overall value of the intelligence gleaned from such sources.

My forthcoming publication of the diary of Ivan Maisky, the ubiquitous Soviet ambassador to London, 1932-43, casts doubt on such assertion. Maisky had a front-row seat at some of the most pivotal events of the interwar era, recording them in the only diary to have been written by a major Soviet official during Stalin’s great terror. In its full version it encompasses more than 1,500 densely handwritten pages.