With such epistolary showmanship to contend with, it might be thought the Duchess would come off a poor second: yet however unbookish, she is a Mitford, and thus in a class of her own. Although at her best, and most engaged, when writing to her sisters (as in The Mitfords, published last year) her letters to Leigh Fermor are a delight, direct, disarming and extremely shrewd.

Briskly dismissing the sophisticated travelogues — “Several bits were v. praiseworthy .?.?. All most educational” — she dashes off highly idiosyncratic accounts of her own experiences, her clear-sighted gaze lighting on some curious incidents and angles: of the Conservative politician Duncan Sandys, for instance: “you know, lives in Surrey & doesn’t hunt. He is interested in Women but probably for only one horrid thing”; of Jackie Kennedy: “She is a queer fish. Her face is one of the oddest I ever saw. It is put together in a very wild way”; and at Covent Garden with the Queen Mother, “one forgets between seeing her what a star she is & what incredible & wicked charm she has got. The Swiss conductor panicked & struck up ‘God Save the Queen’ when she was still walking round the back to get to her box & I heard her say Oh God & she flew the last few steps dropping her old white fox cape & didn’t turn round to see what would happen to it.”

The two of them are not always in agreement. Leigh Fermor, commissioned to write the screenplay of The Roots of Heaven, gives a colourful account of filming in Cameroon with the director John Huston, “wildly bogus, charming, complicated, boastful and ham. I like him very much”. Deborah, on the other hand, did not, deciding after sitting next to him at dinner one evening, “My word he is awful.” Nor did they see eye to eye over Somerset Maugham. Leigh Fermor, taken to stay at the Villa Mauresque by Maugham’s old friend, Ann Fleming (wife of Ian), was horrified by his host: “his face is the wickedest tangle of cruel wrinkles I have ever seen,” he reported with a cold shudder. But this was nonsense, apparently: Deborah’s sister, Nancy, loved him and knew for a fact he was a dear, warm-hearted old man, “[and] all his wrinkles spelt nothing but kindness & benevolence.”

Charlotte Mosley is an expert editor – this is her third volume of Mitford correspondence – and she has made an excellent choice out of the 600 letters at her disposal. Characteristically, Deborah let hers stand exactly as they were written, while Leigh Fermor polished and perfected his.