It's a wild modern story of a vastly wealthy woman at the heart of a hedonistic celebrity set, unconstrained by money or morals, and the happiness that eluded her. It’s ripped from a modern-day glossy magazine—except it takes place among British expats in the East Africa of the Gatsby era, the sinful playground of the rich and irresponsible in the 1920s and '30s. It's written by the wife of the man likely to be Britain's next chancellor of the Exchequer. And it was published in the U.S. this week.

As the Labour government's fortunes fall, all eyes are on the Conservatives and their top duo, leader David Cameron and his right-hand man, George Osborne. So much so that Osborne, who will inherit responsibility for leading Britain out of its recession, was given red-carpet treatment on a recent trip to the U.S., meeting Fed chiefs and Treasury Secretary Timothy Geithner, as the Obama administration prepares for a changing of the political guard in London. Suave, cocksure and impeccably dressed, the Shadow Chancellor is also a frequent figure around smart London parties and arts events.

Now it's Osborne’s wife who is the talking point in social and literary London. Frances Osborne has surprised the U.K. publishing world by producing an unsparing memoir of her great-grandmother Idina Sackville's life and sundry loves. She is at the heart of a rather different elite: the priviligentsia London Notting Hill set who have masterminded the return of the Conservatives as the likely next British government.

Demure and precisely spoken, Mrs. Osborne exudes an utterly respectable heritage: The product of a convent school and an all-female Oxford college, her father was a minister in the Thatcher government. But her maternal forebear was the doyenne of Kenya's Happy Valley set and its fabled seductresses of the reckless White Mischief stories of drink- and drug-fuelled excess, which culminated in the murder of the aristocratic Josslyn Hay—Idina's ex-husband (number three of five), after the jealousies and infidelities took their toll.

Idina left her hard-partying Hooray Henry first spouse and two young sons in 1919 to run off to Africa with a new lover, a lurching pattern of lovings and leavings that established her as a model for Nancy Mitford's character, The Bolter. Osborne has pieced together her subsequent dizzying life from diaries, letters, and family recollections.

In Kenya, Idina tended the beautiful Clouds estate and indulged in a louche merry-go-round of weekend parties, where guests would arrive to be received by the hostess in her green onyx bath. Whisky bottles were placed on the pillows by way of welcome; the after-dinner entertainment was organized by her handing out room keys at random, to decide who would bed whom that night.

She'd fled the iron social constraint of London. “Adultery was accepted in these circles,” says Osborne, “But divorce absolutely was not…So when the unsettled young woman fled her first marriage, she had to give up her two sons. It haunted her all her life.”

I meet Idina's descendant in a Notting Hill cafe, a neat, pale-skinned redhead. If there is a sign of her wild ancestry, it's in her intense blue-eyed gaze: Her great grandmother was no great beauty, but she could do smolder with the allure of an English Marlene Dietrich and her allure to men was unfailing.

Osborne is adamant that being a political wife today won't constrain her. “George does what he does and I do my thing.” That is likely to make her the first spouse to inhabit No.11 Downing Street, the prestigious Chancellor's address at Westminster, while pursuing a career as an internationally published writer.

What drove her to excavate family secrets? So long was the shadow of the Happy Valley anti-heroine seen to fall on her family that Idina's existence was kept secret from Frances as she grew up.

“She was certainly the black sheep of the family and there was a great wariness about being associated with her. Do you really want to link yourself to a woman who had lovers without number? My mother was kept away from Idina, too—the only time her name was mentioned was when my grandmother told her as a little girl that she had died.”

The Bolter rips away this safety blanket. “I do identify with her,” Osborne admits. “She is a mixture of so many things we can understand now. She was one of the early celebrities, living a celebrity life without restraint. She yearned to be free and she made herself irresistibly attractive without being beautiful. But she was also ostracized and separated from her children. Ultimately, she was disappointed by lover after lover.” An odd preoccupation, in the face of it, for a woman who by birth and marriage is at the heart of British Conservative circles. “Not that I intend to follow in her footsteps,” she laughs, "I haven't had more than one husband. Just more than one job."

An early zigzag career took her from training as a barrister (“I absolutely hated it—I had a miserable time and I just felt very lonely") to investment banking, corporate finance, and journalism. She only turned to writing after the birth of her second child. Her marriage to George—the son of the Osborne and Little interiors dynasty—is a match those worldly Edwardian forebears would have approved of.

So I wonder whether she has concerns that the evocation of Gatsby-esque privilege will sit oddly with the new Conservative Party's claim to represent the mainstream of British life?

“It's three generations away from me. I didn't feel it reflected directly on me. There aren't a lot of wife-swapping parties round here—and certainly not in my life!”

The modern trans-Atlantic appeal of Idina is that she belongs in the company of women who break every rule—for good and ill. In New York this week for her publication, Osborne tells me one of the meetings she is most looking forward to is with a Canadian woman, who was the daughter of Idina's fifth and final husband and who contacted Osborne after reading of the book.

“My great-grandmother was the only mother she ever knew, and she remembers her as very tender and loving. So in the end, she did find some of the maternal peace that had eluded her and she's remembered for that as well as the other things. It was a kind of atonement, I think."

Anne McElvoy writes the main weekly political column on domestic and international issues at the London Evening Standard. She has published two books on Germany, The Saddled Cow: East Germany’s Life & Legacy in 1992 and she was co-author of the memoirs of the spymaster Markus Wolf, Man Without a Face.