Mary Wesley gave heart to countless impecunious, unpublished novelists when her first novel, Jumping the Queue, was published in 1983. Wesley was then 70 years old, and the book was a gratifying success, much trumpeted in the press as the work of a promising new talent.

Extraordinarily, Wesley found the energy over the next two decades to fulfil that late-flowering promise. She wrote at least a bestselling novel a year, many of them turned into television mini-series. They sold in vast quantities partly because they were well-written without being too highbrow, and partly because of their racy content. Love blossomed in Wesley’s novels, but so too did sex, robustly engaged in and vividly described. The televising of her best book, The Camomile Lawn (1984) proved particularly fertile ground for love scenes and three-in-a-bed romps.

She herself never held with the much-quoted view that she wrote like “Jane Austen with sex”. “It’s only because I’m so old and it’s usually people who are much younger who go on about it,” was her rather disingenuous reasoning. But the vast amount of time her characters spend thinking about bedroom matters was, perhaps, both her selling point and her one shortcoming as a writer. Occasionally, one could not escape the suspicion that she was going out of her way intentionally to shock.

Though she had lived an unconventional life for a member of her class, it was a life that stood her in good stead when she was concocting plots. She could describe the habits of her upper-middle-class London characters with the intimacy of a former member of the clan.

It was a narrow sort of London, consisting of Kensington, Chelsea and Mayfair, because, as she explained, “in my youth that’s where everybody lived”. Everybody, in Wesley’s fictional world, also seemed to know each other, if not to be related. Some characters popped up in several novels, like Calypso Grant, the sex symbol for a whole generation of men, rather like Anthony Powell’s Pamela Flitton. They were part of an Establishment which, when not staying at each other’s country houses, bump into each other in the Harrods food hall or at the Ritz. But the most attractive characters, like their creator, manage to skate around this world, without ever being bound by it. Like Rose in Not That Sort of Girl (1987), Wesley as a girl had often felt that she would “drown in Establishment soup”.

Wesley’s escape had been to become increasingly and dogmatically left-wing. She was taken by boyfriends to Communist meetings during the 1930s, and she lived much of her life on the knife-edge of poverty. No one could accuse her of not knowing what it was like to be poor. Yet she remained a formidably grand socialist, with the sort of prewar upper-class accent and imperious manner that could cut a swath through shopgirls and waiters. Tilting at the Establishment, at hypocrisy, dullness and snobbery, was her chosen form of political activism.

The daughter of a colonel, Mary Aline Mynors Farmar could also boast the Duke of Wellington on the side of her mother, one of whose family names was Wellesley, a name which she later adapted as her pen name. Her father fought with Kitchener at Omdurman, but because of his glamorous missions abroad he rarely made much contact with his children. She was by her own admission a stubborn, mutinous child.

She had a peripatetic girlhood, following her parents around Europe where she learnt to speak colloquial French, German and Italian. Neither of her parents believed in formal education for girls, and Mary was subjected to nearly as many inexperienced governesses as she was changes of address. When she asked her mother why there were quite so many, she was told: “They couldn‘t stand you, darling.” She did a brief stint at Queen’s College, Harley Street, and at finishing school in Paris, before coming out. Aware of her ignorance among well-educated boyfriends, she enrolled at the London School of Economics on a course of lectures in international affairs.

In 1937 she married an Irish peer and barrister, the 2nd Lord Swinfen. She later described him as “the laziest man” she had ever met: “He used to stroll off to his chambers about 11 and come back in time for cocktails at six, and I don’t think he’d done anything all day except play backgammon.” Mary filled her own time shopping and lunching with friends. It was the sort of conventional marriage so humorously described in Not That Sort of Girl.

Then the war arrived and with it the promise of escape. She was enlisted into the signals deciphering outfit of the War Office, which later moved to Bletchley Park. Though she already had one son and was shortly to have another, her job gave her an excuse to leave them in the safety of Cornwall and to come up to London whenever she wished. Her marriage disintegrated but her social life blossomed. This was the frivolous, sexually liberated London of The Camomile Lawn and Part of the Furniture: “It was terrifying and exhilarating at the same time. There was a lot of love, a lot of laughter, a lot of waiting for news, a lot of grieving. And, my goodness, everyone drank.” As Calypso, her heroine in The Camomile Lawn, explained to her eager young man: “If there’s a war I’ll sleep with you before you get killed. That’s what maidens did in books and I am a maiden.”

By VE-Day, Wesley had numerous admirers, including one serious one: Eric Siepmann, a journalist of German descent. She persuaded her husband to agree to a divorce, but marriage to Siepmann had to wait until he had first located and then divorced his existing wife.

They eventually married in 1952, but in the interim had “lived in sin”, an arrangement that so scandalised her family that she was cut out of all their wills and asked to return the family lace. Her second marriage was blissfully happy in almost every respect, and there was a third son for Mary. Only financially could it be described as a disaster because of Siepmann’s habit of moving the family from London to the country whenever he gave up journalism for a stint of writing unlucrative fiction. Mary had always enjoyed writing and her husband encouraged her to write his reviews for The Times Literary Supplement whenever he had too much work on. In 1968 she had two children’s books published.

Siepmann died in 1970 and Mary was thrown into even greater penury. She moved from her cottage on the edge of Dartmoor to a small townhouse in Totnes. Her income was a widow’s pension, plus £50 a month from a family trust. To survive, she knitted jumpers for friends. The knowledge of what it is to bring children up alone and on a tight budget resurfaced in the plot of Harnessing Peacocks (1986).