Ben Macintyre

The policemen agreed they were living with a most peculiar fellow. One moment he was reading classical literature in the original French and quoting Tennyson, and the next he would be discussing the best way to blow up a train.

'What if?' history is a tricky game, but there is no doubt that the senior planners of D-Day - including Eisenhower and the British general Bernard Montgomery - believed that the Double Cross operation had played a pivotal role in the victory.