The continued existence of the English aristocracy is owed, in large part, to the persistent fascination that those lower down the social spectrum have with them. From Brideshead Revisited to Downton Abbey, via any number of books, films and TV dramas, we all know the constituent parts of the story.

Well-bred, beautiful women will be entangled with equally well-bred but badly behaved men, whose caddish streak betokens either a heart of gold or scarcely contained psychosis, but all will be well with the family estate and the heir apparent by the story’s conclusion. For bonus points, there is often a cameo appearance by royalty, and invariably a dowager lurks somewhere, dispensing bons mots with aplomb. When on screen, she is usually played by Maggie Smith.

The autobiography of Lady Anne Glenconner, then, might appear to be a rattling romp through high society throughout the twentieth and twenty-first centuries, studded with well-worn anecdotes and royal name-dropping. That it frequently causes the reader to put the book down in amazement, before picking it up again to check the veracity of one’s own vision, is quite an unexpected achievement. My notes contain phrases that would make the uninitiated blush, consisting of the likes of “sex show with donkey”, “lesbian orgies in Paris”, “left for dead in a hammock”, “extreme manic outbursts” and so on. If this was bought as a well-meaning Christmas gift for a maiden aunt, I fear that she could well have choked on her mince pie.

Lady Glenconner appeared in fictionalised form in the final episode of the third series of The Crown, played with typical charm and glamour by the wonderful Nancy Carroll. She was introduced as both lady-in-waiting and confidante to Princess Margaret (portrayed by a languid and knowing Helena Bonham Carter, who, as is the way of these things, has her own family connections to Lady Glenconner), and the episode conveyed both the glamour and empty sadness of Margaret’s life.

Yet it is mere soap opera compared to the tales within Lady in Waiting. As Anne writes in the prologue, “I have found myself in a great many odd circumstances, both hilarious and awful, many of which seem, even to me, unbelievable.” You can almost hear the voice of the publisher breathing down her neck: “Can’t you sex it up a bit?”

In fact, sex, in its biological rather than carnal sense, is what dominates the story from the beginning, and the first chapter is entitled “The Greatest Disappointment”. It refers to Anne having been born a girl (“I had tried awfully hard to be a boy”, she writes, the humour barely concealing the poignancy) and thereby forfeiting the right to her father’s earldom, his grand estate of Holkham in Norfolk, and an impressive list of property: “the furniture, the books, the paintings, the silver.”

Her “strait-laced and fastidious” father was a distant, chilly presence, forever fussing about open windows and lavatory etiquette. Her mother, only 19 years her senior, alternated between giving her brisk pep talks and shinning up trees. Yet her family had never been strong on affection. Anne recounts the story of how her great-great-grandfather, a man so stiff and formal that even his wife had to call him Leicester, was surprised to see a baby in the corridor and, upon asking “Whose child is that?”, was informed, “Yours, my Lord.”

Her childhood was in equal parts idyllic and grim. She played with the royal princesses, Elizabeth and Margaret, on roughly equal terms, but was tied to her bed at night by a sadistic governess. When this brute of a woman, presumably in an attempt to curry favour, sent Lady Glenconner a note of congratulations on her engagement, “it triggered the most unpleasant rush of memories and made me physically sick.”

She played with the royal princesses on roughly equal terms, but was tied to her bed at night by a sadistic governess.

The family made it through the war physically unscathed (“we had decided to make Hitler fall in love with us, which, I think about it now, was rather like the Mitfords. But, then, we were going to kill him — which, I suppose, was rather unlike the Mitfords”), although her father developed what would now be known as PTSD, and then Anne was launched into the matrimonial meat market, coming out as a debutante in 1950. This followed a supremely unlikely stint as the only aristocratic female travelling salesman in England, using her feminine wiles to sell her mother’s pottery.

The persistent impression given in Lady in Waiting is that, for all the insulating effects of wealth and prestige, the bubble of aristocracy is a truly miserable one to inhabit, permeated as it is with “characters” who use their positions in society to exploit those around them ruthlessly. In some cases, this is largely benign, as with her uncle-in-law Stephen Tennant.

By the time she met him in the Fifties, the once-beautiful Stephen was a corpulent, grotesque figure, only arising from his bed in June to see his roses, and who enjoyed scandalising Anne with “obscene drawings of sailors in frightfully tight trousers”. And in other instances, as with her husband Colin Tennant, it was anything but.

The late Lord Glenconner emerges from Lady in Waiting as a man who, at best, suffered from severe paranoid delusions that would have seen a less wealthy man institutionalised, and, at worst, was a sadist who took delight in ruining the lives of those around him. He was, of course, possibly both. After Anne caught his eye while serving as a Maid of Honour at the Coronation — a brilliantly described scene which will be invaluable to any future royal historian or biographer — he ruthlessly exploited her fears that, at 22, she would forever be a spinster, and despite his tendency to explode into violent and terrifying outbursts of rage, they married in 1956.

Princess Margaret had Tennant about right when she called him “a fairly decadent fellow”. His idea of romance, after an “awkward and painful” consummation of their marriage, was to take Anne to a revolting Parisian hotel to observe the “intertwined pasty bodies” of a French couple having sex, in the hope that they might make it à quatre. “I found myself saying politely, ‘That’s very kind of you, but no thank you.’”

Paris has unfortunate associations for Lady Glenconner; not only did the indefatigable Tennant also take her to the aforementioned stage show of a man having sex with a donkey there, but, in her eighties, she accidentally attended an orgy at an embassy residence, where, dodging the lesbian sex shows, she haplessly noted that “the bathroom was full of bowls of cocaine, the glass-topped table by the basin criss-crossed with lines”.

The main event in Anne’s professional life, other than dealing with her mercurial husband’s schemes and moods, was to be lady-in-waiting to Princess Margaret, who comes across a good deal better here than in Craig Brown’s magisterial hatchet job Ma’am Darling. She is portrayed as brisk, entitled, given to rather terrifying displays of “fun” (appearing in full Brünnhilde costume to mime along to “Ride of the Valkyries” at a party) and, in extremis, a loyal friend.

When Anne’s second son Henry was diagnosed with HIV in the Eighties, at a time when the room would empty if he walked in, Margaret, the patron of the Terrence Higgins Trust, was a cheering presence, telling funny stories and attempting to lighten the mood.

As the narrative continues, the privations visited upon Anne would not disgrace the Book of Job. Her eldest son Charlie followed the path of too many neglected scions of wealthy families and became a heroin addict, eventually dying of hepatitis C, and Henry, like many other doomed young gay men of his era, died of Aids. Her youngest son Christopher survived a near-fatal motorcycle accident and, one infers from her tactful account, suffered horrendous physical problems thereafter.

Her husband lived down to expectations when, at the end of his life, he disinherited her and their surviving children and instead left what was left of his fortune to his helper, a pleasant, illiterate young man named Kent. Yet she remains optimistic and cheery, concluding that “I have no regrets … I am very much at my happiest and intend to live to a hundred, although still always wondering ‘Whatever next?’”

Lady in Waiting has been a bestseller since its publication late last year. Part of its success can be attributed to its coming out at the same time as the last series of The Crown, and a riotous appearance on The Graham Norton Show where Lady Glenconner more than held her own with Norton’s patented style of fruity innuendo. Yet this remarkable book is also a testament to the quiet gumption and courage that has been instilled in a certain type of English aristocrat for generations.

There is no self-pity to be found here, no moaning about one’s lot in life, but instead a gutsy willingness to make the best of things, whether that is transforming Mustique from her husband’s whim to a haunt of celebrities and royalty (and Boris Johnson), coping with her sons’ illnesses or putting up with the infuriating Colin.

If ever a neo-Corbynite party leads a revolution, and the upper classes are forced up against the wall, one can imagine Lady Glenconner turning to her terrified peers and saying, with just a touch of irony, “Darlings, hasn’t it all been rather lovely? Why would we want to spoil things now, just because some vulgar man has taken a dislike to us?” It is this attitude — in its own well-spoken way as rebellious as punk rock — that permeates Lady in Waiting and makes it succeed as both a riotous social document and a beautifully written account of a vivid life superbly lived.