Take one beautiful young duchess, the youngest and cheeriest of the Mitford sisters. Give her Lismore, a romantic Irish castle plumbed by a modern-minded aunt-in-law (Adele Astaire). Throw in Chatsworth, sometimes described by its chatelaine to her friends and guests as ‘the dump’. Add – don’t even think about sex, for there isn’t a whisper of hanky-panky in a correspondence chastely edited by Charlotte Mosley – one of the most ebullient, erudite and handsome men of his generation. Remind yourself (you won’t need to, since the Duchess and PLF kick off this exchange of letters with tributes to each other’s achievements) that this is the man who, while posted to Crete during the Second World War, captured a German general and held him prisoner for eighteen days, thus winning the undying devotion of all Anglophile Hellenes for ever more. (This really doesn’t seem to be much of an exaggeration.)

Good looks and high spirits apart, the Duchess and the travel-writing hero make an unlikely couple who have nevertheless sustained a loving friendship for half a century. She, he declares to two of their mutual friends in an early letter, is ‘funny,