The codebreakers of Bletchley Park have become the stuff of legend, a stirring tale of the triumph of British brains over Nazi brawn likely to warm the heart of even the most indifferent patriot. For there, positioned halfway between the ivory towers of Oxford and Cambridge, and a mere hour’s train ride from London, a hotch-potch assemblage of pencil-wielding eccentrics and absent-minded academics outwitted the might of the Third Reich, broke its codes, and shortened the Second World War by as much as two years. If the Battle of Waterloo was won on the playing fields of Eton, Britain’s victory in 1945 was sealed in the prefabricated huts hastily erected in the grounds of a Victorian mansion. So delightfully amateurish was it all, so goes the chuckle, that the head of MI6 even had to dip into his own pockets to pay for the building.

Well, up to a point, Lord Copper, up to a point. There’s truth in the legend, but also a lot of tosh. The recently published official history of MI6, for example, neatly dispatches the myth of its chief personally paying for Bletchley Park by revealing that the funds