In the middle of 1944, a series of appar­ently innocuous answers in the crossword puzzle of the Daily Telegraph rang alarm bells at MI5. In the space of a few weeks, the words ‘Utah’, ‘Omaha’, ‘Neptune’, ‘mulberry’ and ‘overlord’ all featured as solutions to the crossword. What was signi­ficant was that these were all codewords relating to the soon-to-occur (and supposedly secret) Allied invasion of Normandy.

Coincidence? Espionage? Treason? The answer, like the solution to a good cryptic clue, is as appealing as it is obvious.