Here Christie offers a biting reflection of the British class system, including a scathing critique of the Foreign Office, which she presents as a boys’ club of ‘purely ornamental’, upper-class chaps.

The joy of The Secret of Chimneys emanates from its supporting cast – the witty and indefatigable Champagne socialist Lady Eileen Brent, the browbeaten Lord Caterham, the hapless Bill Eversleigh, even the overbearing, purple-faced politico George Lomax plays his part with aplomb.