The other Spencer girl

To her astonishment, she finds her dour husband at one end of the table and her friend, Bess, slap in the middle, between them.

Director Saul Dibb cranks up the tension between the women so one feels that wigs may be yanked off and hurled across the marble and corsets ripped apart in an adultery-fuelled rage. It is one of many scenes highlighting the link between Princess Diana and her 18th-century relative, her great, great, great, great-aunt, Georgiana.

The dynamic naughtily brings to mind Diana's famous comment about Camilla's presence in Prince Charles's life, which is even hinted at in the film's tagline for those of us quick to catch on: "There were three people in her marriage".

Does the comparison between the two women bear close scrutiny? Both Lady Spencers were married as young virgins to older, powerful noblemen. Both were among the most fashionable and famous women of their day.

Both husbands had one lasting affair that, on the death of their first wife, was transformed into their second marriage, and both were campaigners: Georgiana political, Diana humanitarian. But here real similarity ends.

While researching the lives of 18th-century women for my own novel set in this period, I realised how difficult life was for women 200 years ago, even if they were rich. Women had so few rights that they were literally served up to their husbands who could then do what they liked with them.

"Daughters are as chickens for the tables of other men," wrote Samuel Richardson in his 1742 novel, Pamela. This attitude went through society, even at the highest level.

Frustration at the inequality of the sexes pours from the letters and journals of married women, who could not vote, own property, or keep their children if they divorced.

In another scene in the film the young, dazzling Georgiana (Keira Knightley) lurches drunk as a skunk across a grand 18th-century room. Her greasy rouge is smeared, the back of her fashionably frizzed wig ablaze from staggering into a candelabrum. As her horrified guests draw back, she stumbles and the burning wig flies from her head. The duchess sits in a deranged haze as her husband, the Fifth Duke of Devonshire (Ralph Fiennes), one of the richest and most powerful men in England, owner of Chatsworth, Devonshire House and Chiswick House, glances down at her and the wig. "Please put out Her Grace's hair," he orders a footman – and walks away.

This icy encounter captures the essence of the film, dramatising the tensions that made Foreman's award-winning biography Georgiana such a popular success in 1998. Dibb's film is a vivid portrayal of the agonising failure of a dynastic marriage between an inexperienced but intelligent teenage girl and an older man, whose main requirement from his wife was a male heir and not a whiff of scandal.

As Foreman says: "It was an unwritten rule in 18th-century society that a wife should present her husband with a legitimate first son – whatever she chose to do afterwards."

Georgiana was born at Althorp into the vastly wealthy Spencer family, and one of the few men in England who was a suitable match was the supremely wealthy William Cavendish, Duke of Devonshire. Given that women were effectively bought and sold between fathers and husbands, and were legally under their "protection" (control), Georgiana's dowry and pedigree put a very high price tag on her.

As a Gainsborough portrait shows, she was a genuine beauty with tremendous style who can certainly be considered a fashion icon. What she wore was immediately engraved and circulated in pamphlets and the new fad, newspapers, and copied by women's dressmakers everywhere.

The day before her 17th birthday, with no previous sexual experience, Georgiana was married to the 26-year-old duke. She had not been married long when she realised that the duke, whom she nicknamed Canis (Latin for dog), was not spellbound.

The lonely young woman took up reckless gambling and made friends with Lady Elizabeth (Bess) Foster (played by Hayley Atwell in the film) who, separated from an unhappy marriage, was a similar age. The young women became inseparable. Georgiana showered Bess with money and gifts and invited her to stay. Living with a cheating husband in enormous houses, uprooted from her family, this is not surprising. Nor is it particularly surprising that the duke and Bess became lovers.

More unusual, for the rest of Georgiana's life, they lived in a mnage trois. But there were far more than three people in this marriage. Bess had other lovers, including the Duke of Richmond and Count Fersen of Sweden; while Georgiana had an affair with the Duke of Dorset, followed by a passionately open (and therefore very dangerous) one with Charles Grey, later the Prime Minister, by whom she had a daughter, Eliza.

Georgiana's relationship with Bess is well documented, for Georgiana was a fluent letter-writer who expressed her feelings in a way that is now rare. At the time, cultivating a "romantic sensibility" was fashionable (think Jane Austen).

Women made close and demonstrative friendships. Thrown on each other's company day in, day out, without gainful employment, they relied on each other for diversion.

History echoes from Georgiana's to Diana's life but it does not repeat. Today, it is difficult to grasp an 18th-century woman's lack of rights and opportunity. Few jobs were open to women. Poor girls could earn a pittance in service, or as a paid companion.

Wealthy women, on the other hand, had very little to do, condemned to stultifying superficiality and inaction at home. Educating girls was looked at askance. So, having been taught almost nothing, a young wife was doomed to focus on clothes, pastimes and pleasing her husband.

The polemic feminist of the day, Mary Wollstonecraft (whom Horace Walpole called "a hyena in petticoats"), considered women's brains a wasted resource; so much unmined gold. Her treatise, A Vindication of the Rights of Women, pleaded for women to be freed from being mere decorative baubles, where the very boredom that encased them led to licentiousness and dissipation.

Furthermore, in a century when women were treated as commodities, men could easily have sex outside marriage: with married women, with servants, with high-class courtesans or low-class tarts. But if a wife did the same and was caught, she risked being left penniless by divorce, ostracised by her friends, and her children taken from her forever – all with no avenue for legal redress. A wife's adultery was seen in legal terms as a sort of treason and could be punished by death.

She could not easily divorce her husband for adultery, although she could be divorced for the same offence. In such a case, husbands got automatic custody of the children.

The duke uses this lever to separate Georgiana from Charles Grey, pointing out – with the full force of law behind him – that if she runs away, she will never see her children again. The terrible rigour of the law should not be underestimated: until 1786, death by burning was still on the statute books.

In one scene in the film, the duke rapes Georgiana as punishment for wanting to continue her relationship with Charles Grey while he continues his with Bess. "Are you his whore?" he bellows, before pursuing her – in one of the most menacing walks ever seen on film – down a long, long, long corridor, to her bedroom.

One area where the film short-changes its heroine is by minimising her political influence and her part in bringing the Whigs to power. In real life, Georgiana was an important, committed political campaigner for her distant cousin, Charles James Fox, who led the party. She appeared on the stand with him at the hustings, drumming up support in her eyecatching outfits. The Devonshires used their wealth to fund the party and hold endless receptions. Georgiana also advised both the Prince of Wales and Fox, and her advice was taken.

It's a sad truth that if Saul Dibb's film reveals any true link between these two troubled women, Diana and Georgiana, it is that, after 200 years, we still focus on a woman's beauty, love affairs, and the scandal that attaches to her, rather than anything else they might have achieved in the world.

The Duchess is released on 5 September. Philippa Stockley's Booker-nominated novel A Factory of Cunning is out now, published by Abacus.