Eleventh Hour (The Carstairs Affairs, #1)

Pursuing a ruthless enemy who wants to throw Europe back into the horrors of the Great War, Briers Allerdale returns to 1920s London to warn his masters in the Secret Intelligence Service of a dangerous anarchist plot. He will need back up – but of a very specialised kind.

Borrowed from the cipher department Miles Siward moves into a 'couples only' boarding house, posing as Allerdale's 'wife'. Miles loves allowing his alter ego, Millie, to spread her wings but if Miles wants the other agent...