"'Between Silk and Cyanide' by Leo Marks."
IIRC he was the SOE crypto expert (he also wrote the Times Crossword).
His family owned the bookshop at 84 Charing Cross Road described by Helen Hanff.
His (anonymous) poem is the one used in the film "Carve her name with pride."
Some of the SOE codes had keys that were poems. To avoid the Germans working out if they were being pulled from a book of poetry (and brute forcing any transmission they recorded) he made them up.
Sounded like a bit of a character.