So exactly how many men has Diana Athill slept with? Her eyes, still clear at 90, brighten with laughter. “I don’t know,” she says. “I’m not going to count them now.” Her accent is patrician, but the vulgarity of the question does not offend her.

It’s a mug’s game trying to shock Miss Athill. After 45 years as one of London’s most brilliant editors – she published the early work of Norman Mailer, Philip Roth, John Updike and Margaret Atwood – she has, late in the day, become a much-praised author in her own right.

Since turning 70 she has written four unsettling, incisive books about her own life. Her calling card is unsparing candour: anyone who imagines that sexual intercourse began in 1963 should read Athill’s account of her trysts in wartime London in her first superb volume of memoirs, Instead of a Letter. She had been jilted at the age of 22 by her fiancé and – cheated of her destiny as wife and mother – she filled the emptiness with promiscuity. “Lack of energy prevented me from ranging about in pursuit of men, but if they turned up, I slept with them.”

Her lovers included André Deutsch, with whom she co-founded the publishing house of that name. Her unsuccessful suitors included one of its stars, V S Naipaul, who came to dinner and tried to kiss her. Having seen his self-importance at close quarters, she found him easy to fob off.

Maybe Naipaul was unlucky, for the elegant white-haired lady sitting among the needlepoint and watercolours of her flat in Primrose Hill, north London, was willing to kiss just about anyone who asked nicely. She was good at being the Other Woman. Did she think about the betrayed wife? “I didn’t. But I do have to say I never slept with anyone whose wife I actually knew.”

She herself claims “not to be possessive by nature”: certainly, when her longest relationship – with Barry Reckord, the Jamaican playwright – ran out of sexual steam she was happy to welcome his new lover, Sally, into their flat. Sally is now one of Athill’s dearest friends, almost a daughter: “I will probably live with her when I’m really old.” As for Reckord, after years of illness he has recently been whisked home to Jamaica by his family.

This is all fabulously liberated, if unorthodox. “True,” concedes Athill. “But I don’t see it as virtue, I see it as incredible luck.” The same luck led her into her last affair, with Sam, a handsome Grenadan whom she visited weekly for sex well into her sixties. He was married but, as they shared only a love of sex, this was fine.

I find her claim that nobody should expect loyalty from a spouse chilling. In response she quotes an interview with a retired prostitute who insisted that any man will pick up sex where he can. “And you know, I think she was probably right. It’s a matter of opportunity very often. The interviewer was shocked, just like you. But I’m with the prostitute on that one.

“I think it is possible – probable – for a husband to go on liking his wife very much, while having the occasional flutter that she doesn’t know about and that has done her no harm. With men, you know, their sex drive is much stronger and, you know, it’s much less important.”

So is she male in this respect? “On the whole, I behaved rather like a man. I was just very lucky I got away with it, but that I think was quite unusual,” she replies. It’s not so unusual now. “Yes, and that is generally proving itself to be a mistake.”

Aha! A regret! There’s something about Athill’s confident upper-class ability to rise above the more suburban human emotions – jealousy, possessiveness, infatuation – that makes teasing irresistible.

“No, not a regret. I had a lovely time,” she replies, a note of asperity in her voice. This is true, up to a point. Athill has said in the past that for the first 20 years of her adult life she was haunted by what she calls “a hidden sense of failure . . . because I hadn’t succeeded in marrying, because I hadn’t had children”. It was only in 1963, when she wrote Instead of a Letter – which told of her idyllic country house childhood, the jilting, the rackety years – that the sense vanished. Now she thinks all that heartbreak was “not a bad thing”: her life has turned out pretty well.

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