“I’m writing a book about us when we were little,” Nancy Mitford wrote to her sister Jessica on April 13, 1945. “It’s not a farce this time but serious — a novel, don’t be nervous!” The novel was The Pursuit of Love and Nancy’s family had every reason to be nervous.

The seven Mitfords — Nancy, Pamela, Tom, Diana, Unity, Jessica and Deborah — would become the six Radletts — Louisa, Linda, Matt, Jassy, Robin and Victoria. The Mitfords’ mother, Sydney, Lady Redesdale, always “Muv”, became the thinly disguised Aunt Sadie and their father, David Freeman-Mitford, 2nd Baron Redesdale, “Farve”, became Uncle Matthew.

If you’ve read the book, I’ll only have to write the words “child hunt”, “entrenching tool” or “thin end of the wedge” for Uncle Matthew to appear before you, roaring and furious. The newspapermen in The Pursuit of Love make Uncle Matthew out to be “something between Heathcliff, Dracula, and the Earl of Dorincourt [the irascible, disinheritance-happy grandfather in Little Lord Fauntleroy].” Uncle Matthew rather relishes the fight with reporters. The real-life Muv and Farve were less keen on pressmen. “Whenever I see the words ‘peer’s daughter’ in a headline,” Lady Redesdale said, “I know it’s going to be something about one of you children.”

The Mitford sisters were irresistible then and fascinate now. The glamour, the scandal, the teases, the letters! Stop me if you’ve heard this before, but here’s a quick whisk through: Nancy the novelist and wit-for-hire, Pamela the cook and countrywoman, Tom the only son, who was killed in Burma in the Second World War, Diana the beauty who married the fascist Sir Oswald Mosley, Unity who took up with Hitler and the Nazi party and attempted suicide when war was declared, Jessica the Communist who eloped to Spain with her cousin, and Deborah who married the Duke of Devonshire and kept chickens at Chatsworth.

The Pursuit of Love has become a foundation stone of the Mitford myth: running-away funds, games of patience and, of course, the Hons Cupboard. The challenge for any Mitford biographer is to distinguish between what is real, what fiction and what exaggeration for wicked effect. The writer DJ Taylor puts the enduring appeal of the book in part down to “its status as a kind of Mitford family acrostic, Nancy projecting some of her far-from-straightforward feelings about Muv and Farve and her sisters into fiction, the Mitford ‘tease’ being used to disguise/anaesthetise/burlesque lashings of bygone hurt”.

The narrator isn’t Nancy herself, but Fanny, an only child and an awed and adoring cousin to the Radletts. Fanny’s mother abandoned her daughter as a baby and has abandoned every lover since. She is known by all as “the Bolter”.

The book has been adapted three times for the small screen. First by the playwright Simon Raven for ITV in 1980 with Judi Dench as Aunt Sadie. Then by the novelist Deborah Moggach (Tulip Fever, Best Exotic Marigold Hotel) for the BBC in 2001 with Celia Imrie as Aunt Sadie, Alan Bates as Uncle Matthew and Rosamund Pike (too beautiful) as Fanny.

Now comes a new Pursuit of Love (BBC again) with a screenplay by Emily Mortimer. The three-part series received a not inconsiderable publicity boost last summer when Lily James (Linda) and Dominic West (Uncle Matthew) were photographed in a series of off-screen, on-scooter clinches. Emily Beecham is a lovely, subtle Fanny, but still too pretty. Television abhors a dowd. Andrew Scott (Moriarty in Sherlock) plays the vamping Lord Merlin, aesthete extraordinaire, and Mortimer has cast herself in the plum part of the Bolter. The first episode is clever and charming, but all the best lines are Mitford’s and all the duff ones newly invented.

Women divide, I reckon, into Fannys and Lindas. Fannys are not unromantic, but they are realistic. They tend to be attractive more than devastating. They remember birthdays, appointments and clean handkerchiefs. Lindas are rare, gorgeous, flighty and liable, when it comes to men, to make the same mistakes twice, thrice and four times over. Linda is the original hot mess. Lindas sometimes mellow into Fannys, but Fannys rarely, if ever, rev up to be Lindas. (I am a Fanny. And so, most likely, are you.)

The Pursuit of Love has become a foundation stone of the Mitford myth.

Nina Stibbe, an author in the Mitford comic tradition, also points out that Linda’s “amusing carelessness, as a parent, to poor ugly, badly named Moira — and the fact that she goes unpunished for it by the other characters — emboldened a whole generation of women. The parenting in general: hilarious.”

Nancy’s working title for the book was “Linda” and really it is Linda’s story. Nancy wrote what would become The Pursuit of Love (her friend Evelyn Waugh suggested the title) in a storm of inspiration in the early months of 1945. It was her fifth outing after Highland Fling, Christmas Pudding, Wigs on the Green and Pigeon Pie. These first four novels, if not quite the “farces” she described to Jessica, are witty enough, although slight. The blade isn’t yet sharp. The Pursuit of Love is all edge and gleam. Nancy couldn’t remember if she started writing before reading Waugh’s Brideshead Revisited or after. In any case, she wrote to Waugh: “It’s about my family, a very different cup of tea, not grand and far madder.”

She was in love when she wrote it. When Linda meets Fabrice, Duc de Sauveterre after two failed marriages, both ending in bolts, she is “filled with a strange, wild, unfamiliar happiness, and knew that this was love. Twice in her life she had mistaken something else for it; it was like seeing somebody in the street who you think is a friend, you whistle and wave and run after him, and it is not only not the friend, but not even very like him. A few minutes later the real friend appears in view, and then you can’t imagine how you ever mistook that other person for him.”

The model for Fabrice was Gaston Palewski. Nancy called him “the Colonel” or “Col” after his rank in the army. He had served in the French air force and was directeur de cabinet of General Charles de Gaulle’s government in exile in London. They met in the garden of the Allies’ Club in London. The Colonel was enchanted by Nancy’s stories about her eccentric family. “Racontez, racontez,” he would say. Tell, tell. Fabrice says the same to Linda.

The Mitford sisters were irresistible then and fascinate now. The glamour, the scandal, the teases, the letters!

The historian Lisa Hilton, the author of The Horror of Love: Nancy Mitford and Gaston Palewski in Paris and London, has described the Colonel as having a face like “an unpeeled King Edward”. Fabrice is “short, stocky, very dark”. In the new series, Fabrice is played by the definitely too dishy French-Moroccan actor Assaad Bouab (Hicham Janowski in Call My Agent!).

Nancy worried that she could never better the book’s two leading males. To Waugh she wrote: “Also you see what nobody else seems to that having used up Farve & Fabrice I am utterly done for — all the rations have gone into one cake. Now what?”

Fabrice is not the Colonel in all respects. Nancy’s biographer Selina Hastings notes a spot of wish fulfilment when Fabrice tells Linda he loves her. “But,” Hastings writes, “as a testament of her heart and mind it is true to the last love letter.”

When Fabrice fills Linda’s Paris hotel bedroom with flowers, Linda is thrilled, if disbelieving. “Really,” she thinks, “this is a very penny-novelettish seduction, how can I be taken in by it?” Nancy was never penny-novelettish. “I doubt,” Taylor says, “if Nancy Mitford ever wrote a romantic line in her life.” No one is crushed against manly chests, no one is swept off their feet. Fabrice doesn’t pick Linda up and twirl her round, but he does carry her luggage and call her a taxi. More useful, really, to a girl. There is no melodrama.

Linda’s marriages aren’t disasters, but they are dull. Boredom was Nancy’s greatest fear: being bored, boring others. Linda’s first husband, Tony Kroesig, a banker and Conservative MP, is boring and pompous; her second husband, Christian Talbot, a Communist, is boring and earnest. (Nancy’s one and only husband, Peter Rodd, “Prod”, was boring by all accounts.)

The Pursuit of Love was published on December 10, 1945, printed on poor paper “in conformity with the authorised economy standards”. The book was a colossal success. The public were weary with war, wearier still with the short-rations peace. Churchill had fallen to the socialist Clement Attlee. “Amid the ensuing gloom,” Nancy’s friend Harold Acton wrote, “with mediocrity vengeful and triumphant, The Pursuit of Love was like a gloom dispersing rocket.” Farve, the real Uncle Matthew, it was reported: “Sat with his nose in the book & grunted out various corrections: ‘Never got the stock whips in Canada, a bloke from Australia gave them to me’ & so on. He was delighted with it but cried at the end.”

The book sold 200,000 copies in its first year. Nancy’s publisher, Hamish Hamilton, thought she might make as much as $1,000. In the first three weeks of publication she made $1,100 and in six months more than $9,800.

“More thousands for the book,” Nancy reported to Diana in April 1946. “Two more to be exact. So I’ve simply let go everything & buy whatever takes my fancy, it is heaven.” She bought truffles, she bought swansdown powder puffs, she bought (and bought and bought) Dior. “Quel joli coup de taille!” the seamstresses cooed. (“What a slim waist!”) It was the New Look age. Nancy was outraged when she had to queue at her couturier. “Impossible to get inside the building,” she complained to her friend and former employer Heywood Hill, the owner of the bookshop of that name. “I had to use INFLUENCE to be allowed to order. Why is everybody so rolling — they can’t all have written Pursuit of Love.”

Nancy heard that the Duke and Duchess of Windsor (Edward and Mrs. Simpson) were giving copies to everyone for Christmas, “which tickles me very much”. The Colonel was “greatly tickled at his own portrait”. It was a favorite word. Ticklish — that’s it. A tickle is pleasure and pain. No spoilers, but if you can read the last two pages without weeping then you are a terrific Counter-Hon. I’m with Farve. It is a sad book. Not sad-sad, more lovely-land-of-might-have-been sad.

But don’t let me do down the funny. Recently reading the book for the fourth or fifth time I giggled, I chortled, I snorted. “Doesn’t sound much like work to me,” my husband said. “I don’t make those noises when I’m working.” True. But that’s anti-fraud legislation for you.

In his journals, the novelist Anthony Powell wrote: “Nancy could be funny, very hospitable, at the same time she was hard, immature, ununderstanding, making one think of an unkind, unhappy schoolgirl, perhaps what Nancy remained all her life.”

Ununderstanding in person, perhaps. Ununderstanding in sympathies, possibly. However, she understood absolutely the pursuit, the illusions and the delusions of love. The great last line of the book is spoken by the Bolter. Talking of Linda and Fabrice, Fanny tells her mother: “He was the great love of her life, you know.” You may know the next bit. All together now. “ ‘Oh, dulling,’ said my mother sadly. ‘One always thinks that. Every, every time.’ ”

Laura Freeman is the author of The Reading Cure: How Books Restored My Appetite