“It was almost as though, early in life,” writes Brown, “she had contracted a peculiarly royal form of Tourette’s Syndrome, causing the sufferer to be seized by the unstoppable urge to say the wrong thing.” He supplies many instances, of which the most wince-making is probably the time she asked an architect who had been disabled since childhood: “Have you ever looked at yourself in the mirror and seen the way you walk?”

Sometimes the wrong thing was said by others. Michael Holroyd sat next to her at dinner, and was amused by her imitation of Edna O’Brien. When she began another impersonation, he dutifully laughed: “If I may say so, Ma’am, I think that’s your funniest yet.” She was speaking in her own voice. Brown inevitably devotes much of his narrative to the Princess’s unhappy love life. He rehearses her engagement to the divorced Group Captain Peter Townsend and her marriage to Antony Armstrong-Jones, later Lord Snowdon, whom Brown calls “a sort of upper-class Mod” and Kingsley Amis called “a dog-faced tight-jeaned fotog of fruitarian tastes”.

By 1950, Margaret had been adopted as a “national sex symbol”. James Lees-Milne wrote to John Betjeman that he found her “very, very, very frightening but beautiful and succulent like Belgian buns”. When she presented Betjeman with the Duff Cooper Prize in 1958 he was reduced to silent tears, moving his friend Maurice Bowra to write a parody of Betjeman’s “In Westminster Abbey”:

“Green with lust and sick with shyness,

Let me lick your lacquered toes.

Gosh, O gosh, your Royal Highness,

Put your finger up my nose…”

“It was in the early 1950s,” begins one chapter arrestingly, “that Pablo Picasso first began to have erotic dreams about Princess Margaret. Occasionally he would throw her elder sister in for good measure.” He was obsessed with her for more than a decade, and seriously wanted to marry her. This inspires Brown’s next chapter, from a counter-factual biography of Picasso: “Picasso made repeated clandestine visits to Kensington Palace, often sporting a false-nose-specs-and-whiskers mask…”

Jeremy Thorpe also hoped to marry her, and when her engagement was announced sent a card to a friend: “What a pity… I rather hoped to marry the one and seduce the other.” Another chapter parodies a feature in Hello magazine, entitled “Lord Thorpe of Barnstaple and HRH The Princess Margaret, Lady Thorpe, Welcome Us Into Their Beautiful Home”.