Enemies Within: Communists, the Cambridge Spies and the Making of Modern Britain by Richard Davenport-Hines

It was at this time [1937] that a Cambridge luminary, the novelist E.M. Forster, wrote a credo that has been lampooned, truncated in quotation and traduced by subsequent writers. His remarks in their entirety carry a message of individualism, conscientious judgement and anti-totalitarianism that might have been a text for Whitehall [the center of government for the United Kingdom] values in the 1930's.  "One must be fond of people," said Forster, "and trust them if one is not to make a mess of one's life, and therefore it is essential that they should not let one down.  They often do."  Writing in 1939, when totalitarian nationalism was rampant, Forster continued: "Personal relationships are despised today.  They are regarded as bourgeois luxuries, as products of a time of fair weather which is now passed, and we are urged to get rid of them, and to dedicate ourselves to some movement or cause instead.  I hate the idea of causes, and if I had to choose between betraying my country and betraying my friend, I hope I should have the guts to betray my country."
--Richard Davenport-Hines, Enemies Within (William Collins 2018)

I was a graduate student in art history in 1978 when Margaret Thatcher announced that Anthony Blunt (an expert on Poussin, one of my favorite painters) was the fifth of the Cambridge spies; we mostly shrugged our shoulders, just as I did when James Weinstein, founder of "In These Times," said he thought Alger Hiss and the Rosenbergs were guilty because he would have done the same thing if he had been offered the chance.  Davenport-Hines's book is a bit of a slog in the beginning, as he admirably traces communist spies in England back to the Russian revolution and analyzes the motives and actions of Blunt, Burgess, Cairncross, Maclean and Philby, as well as many others.  

Backed up with an extraordinary amount of evidence and thoughtful reflection Davenport concludes that it was gender loyalty more than class loyalty (the spies were mostly middle class) that caused co-workers in MI5 to give each other the benefit of the doubt; married women were banned from the civil service until 1946 and the Diplomatic Service until 1973, though there were some outstanding exceptions who received waivers. The fact that a couple of the spies were gay and/or dipsomaniacs is irrelevant to their motives, which were almost exclusively political and began when the Soviet Union was England's ally in WWII.

Among Davenport-Hines's conclusions:
I suggest that the preoccupation with class loyalties and social exclusion, which has dominated histories of communist espionage in Britain, is  a species of self-serving Marxism which relies on illusory or falsified readings of the English class system.