Hallie Rubenhold bills this as “one of the first celebrity divorce cases”. It was certainly sensational. Sir Richard Worsley was comptroller of the King's household, an MP and governor of the Isle of Wight. His wife, co-heiress to the manor of Brompton (now site of the Victoria and Albert museum), was said to have taken 27 lovers - several of whom were called to court as witnesses. Her doctor testified that he had treated her for a “Venereal Disorder”, which, it was implied, she had caught from the Marquess of Graham.

The Worsleys' wedding in 1775 had caught the public imagination. Sir Richard married Seymour Fleming at Harewood, the smart new Yorkshire palace where her mother, Lady Fleming, was chatelaine. “Tuneful hail the virtuous fair,” chirruped the Hampshire Chronicle, “Happy, happy be the pair.” But the two were miserable from the outset, and Seymour, after giving her husband a quick heir, was extravagantly unvirtuous. In 1781 she eloped with an Isle of Wight neighbour, George Bisset. Sir Richard sued Bisset, in 1782, for “criminal conversation” with Lady Worsley, demanding £20,000 in damages - a fabulous sum. When the jury granted him one shilling, he was a laughing stock.

Rubenhold, the author of The Covent Garden Ladies, a racy study of 18th-century London prostitution, has an eye for an antique story. An art historian, she was drawn to Joshua Reynolds's portraits of the ill-matched pair. Worsley, dressed as commander of the South Hampshire Militia, looks conventional, decent, even vulnerable. On the other hand, the companion portrait of his wife - a poised, sharp-featured woman in matching militia braid and scarlet riding habit, whip in hand - is strangely disturbing.

Seymour made an early bid for independence. She was said to be a model for Lady Teazle in Sheridan's play The School for Scandal (1777). But not even naughty Lady Teazle would have got up to such japes as Lady Worsley. At a 1778-79 New Year's ball at Harewood, she flung the male guests' clothes (“particularly their breeches”) out of the window. Later in the month, with two reckless accomplices, she set fire to militia flags in a Leeds pub and vandalised a local grandee's library. Rubenhold opines, portentously, that this may have been a cry for help.

Yorkshire mischief escalated to London scandal. What was extraordinary about the 1782 trial was that it was not Sir Richard who brought peer after amatory peer to the witness box, but Bisset. His defence, which can only have been devised by a vindictive Lady Worsley, was that her husband had been tirelessly permissive. When at 4am he found Viscount Deerhurst in her dressing room, all Worsley said was, “Deerhurst! How came you here?”

Worse, Bisset's counsel suggested that Worsley promoted his wife's indiscretions; he was a voyeur. Central to the evidence was an incident in 1781 - captured by James Gillray in a brutal cartoon - when Worsley hoisted his wife's lover up to a window to peep at her naked in a bath-house. “Seymour! Seymour!” cried Worsley. “Bisset is looking at you.” The jury decided that, while Worsley might not have solicited the “debauchery” of his wife, he had clearly condoned it.

Lady Worsley's Whim takes its subject from a zesty period: even as Seymour was organising a post-trial “ball to all her beaus”, Britain was losing the American war of independence and France was preparing for revolution. Rubenhold is sure-footed in her research (though she eschews specific references), but not so secure on aristocratic nomenclature, and her editor could have administered a brisk dose of commas to her text. Her special forte is rakes and roués: her depiction of Coxheath Camp, where the country's militias gathered for months as a glorified Home Guard, idling and fornicating, is deliciously lurid.

She presents herself as “an archeologist piecing together an ancient mosaic”. At the centre of her pavement are the transcript of the trial and the numerous anonymous pamphlets that followed it, many of which are as saucy as A Peep into Lady W!!!!!y's Seraglio, Gillray's illustration of a staircase of lovers waiting to pleasure her ladyship in her chamber. Out on the edge of the pavement, an untidier, vaguer place, Rubenhold has been diligent in collecting fragments, pursuing the Worsleys in the dismal aftermath: Sir Richard in his five-year exile on the grandest of grand tours (when he assembled a collection of Greek antiquities only surpassed by that of another scandalous cuckold, Lord Elgin), and Seymour, utterly disgraced, left by her lover, forced abroad and, only with her husband's death in 1805, finding some sort of resolution, retrieving her fat inheritance and marrying again.

Yet, however ably they dig, archeologists produce only a partial history. What is lacking in Rubenhold's story - because, as she suggests, any papers were deliberately destroyed - is Lady Worsley's authentic voice. All we sense of her is manic, inexplicit, a boiling frustration. That frustration and silence communicate disquietingly to the modern reader.

Lady Worsley's Whim by Hallie Rubenhold
Chatto £25 pp320

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