Normally an obituary in The Times would provide a framework for a biography of an important person in any given field, but that simply wasn't true of the one written for my boss at the British wartime codebreaking base at Bletchley Park. This was the wonderfully eccentric but outstandingly brilliant Alfred Dillwyn Knox, known to his many friends and admirers simply as ‘Dilly’.

George Steiner, the American writer and philosopher, has described the codebreaking achievements that took place at Bletchley Park as ‘the single greatest achievement of Britain during 1939-1945, perhaps during the 20th century as a whole’. If that is true, then Dilly’s own achievements must be ranked among the greatest in their own right.

Dilly’s work on the various Enigma ciphers was certainly among the most important and significant carried out at Bletchley. Enigma was not one single cipher machine, as is often suggested, but a family of many different ciphers and it was Dilly and his research section, of which I was a proud member, who were asked to find a way into each new cipher as it appeared.

The failure of his obituary in The Times to do him justice when he died in early 1943 was caused by the absolute secrecy surrounding the work on Enigma. The obituary mentioned that his father was a former Bishop of Manchester; that his brother was Monsignor Ronald Knox, a famous Catholic theologian; and that another brother, ‘Evoe’, was editor of Punch. It also mentioned his work as a Classicist reconstructing the mimes of the Greek poet and playwright Herodas.

What it could not mention was that he was one of the leading members of Room 40, the Admiralty’s celebrated codebreaking section during the First World War, broke Bolshevik ciphers during the 1920s and 30s, and Enigma ciphers during the Spanish Civil War and Second World War. What it would certainly not have been possible to mention, even without the understandable secrecy, was that Dilly’s greatest triumph had not even taken place when the obituary was written.

Shortly before he died, in great pain from the cancer, Dilly broke the Enigma cipher used by the German secret service, the Abwehr. It was this that allowed MI5 and MI6 to manipulate the intelligence the Germans were receiving through the Double Cross System and fool them into leaving too few troops in Normandy to counter the allied landings.

Now that many more previously secret records have been released into the archives, I have at last had the chance to give my old boss the credit he deserves. I felt a strong sense of déjà vu in seeing once more the same secret enemy messages that we handled over sixty years ago, but then the secrecy was such that even I was unaware of the effect Dilly’s work had on the allied success in the war. I was determined in writing this book to ensure that what Dilly did was never forgotten.