Early in 1940 Hugh Trevor-Roper cracked a German secret service cipher while lying in his bath during an air raid. A brilliant young don at Christ Church, who would later become Oxford’s Regius Professor of Modern History, Master of Peterhouse, Cambridge, and a peer, Trevor-Roper had been recruited to work in the Radio Security Section (RSS) of Military Intelligence. First housed in Wormwood Scrubs Prison, it included among its tasks the interception of signals sent from the Continent to German spies in Britain. This brought the RSS into conflict with the Government Code and Cypher School at Bletchley Park, which wanted a monopoly on code-breaking. It found Trevor-Roper, who advocated cooperation between the intelligence agencies, especially obnoxious. For a time he was denied entry to Bletchley, whose deputy head said that he seemed ‘unable to speak the truth even by accident’.