The novelist Mary Wesley never forgot the night of 26 October 1944. She was then 32, locked in a loveless marriage to ‘a perfectly nice but remarkably boring’ barrister, Lord Swinfen, and was dining at the Ritz with a friend from MI6 — she had worked there in April 1940, decoding the positions of German regiments — when she looked up and saw, seated at another table, the Royal Marines captain whom she had met only a few hours earlier at Les Ambassadeurs.