To connoisseurs of the early British spy fiction published during the decade or so before the First World War, purity constitutes part of the nostalgic appeal. Look back to the dashed-off novels of Headon Hill or E Phillips Oppenheim – or even to Erskine Childers’s more thoughtful The Riddle of the Sands – and you gaze on a simpler version of the Great Game, unclouded by the murky ambiguity that would later descend upon the hunched shoulders of the Ambler or Le Carré spook. 

Those novels, of course, were themselves always concerned with matters of purity. Decrying the presence of a network of untrustworthy foreigners residing on English soil, they hinted at the desirability of a purification of the body politic; lamenting the lethargy and unpreparedness of the English male, they proposed a regime