This is a well-written, scrupulously researched and argued account of an enduring mystery that neatly illustrates the haphazard interactions of politics, bureaucracy and history.

In 1924 Grigori Zinoviev was head of the Communist International, the propaganda arm of the Soviet regime. A letter in his name, dated 15 September and addressed to the Communist Party of Great Britain (CPGB), urged comrades to foment insurrection in the armed forces and among munitions workers while publicly supporting the ratification of an Anglo-Soviet trade treaty and a large loan to Russia.