"There were three of us in this marriage, so it was a bit crowded," Princess Diana once famously remarked about her union to Prince Charles, and the additional presence of a certain Camilla Parker Bowles. 18th century royal Georgiana Spencer was in a similar predicament. To make the comparison to the Diana-Charles-Camilla love triangle isn't a stretch. Both the princess and Georgiana were popular public figures - plus Georgiana is Diana's great-great-great aunt.

Based on the biography Georgiana: Duchess Of Devonshire by Amanda Foreman, The Duchess charts the troubled, fascinating life of a woman who was married to "the most powerful peer in England". Portrayed by the reigning queen of corsets, Keira Knightley, the film focuses on Georgiana's uneasy relationship with her taciturn, distant husband - the much older William Cavendish, Duke Of Devonshire (Ralph Fiennes). The marriage, arranged by Georgiana's mother (a splendid Charlotte Rampling) and Cavendish himself, smells like a business deal. Lady Georgiana will graduate to duchess status, but she's been drafted into the marriage to provide Cavendish with an heir. The vivacious Georgiana is initially intoxicated by the proposal, though the union is doomed from the very beginning. As the struggle to produce an heir builds, Georgiana meets the comely Bess Foster (Hayley Atwell), and this connection shapes the events that follow. Knightley and Atwell play off each other brilliantly to reveal a contradictory, complex relationship, with a hint of lesbian love. It's ultimately Bess who plays the Camilla Parker Bowles role in Georgiana's life.

The Duchess doesn't reach the elevated heights of, say, Dangerous Liaisons or one of the many sumptuous European period pieces that have been released over the years. The film's rich settings certainly have an authentic old world feel, but this BBC Films co-production also has an unwelcome, over-polished Hollywood veneer, despite being shot on location in stately, vast English manors. Yet some superbly crafted scenes come close to greatness - the dinner where Georgiana is the only female at a long table lined with men takes you back in time, as does the tragically amusing scene where a worse-for-wear Georgiana's wig catches fire.

There are other minor quibbles. As Cavendish, Fiennes is perfectly cast as the unlikeable Duke who we ultimately find some sympathy for. Cold but quietly seething underneath, he has rare moments of warmth and compassion, but treats Georgiana horrendously. The Duke is a key supporter of the Whigs (who evolved into Britain's Liberal Party. Fashionable Georgiana was a trendsetter, a gambler and a political animal - a small 'l' liberal. But her political intellect only comes across in snatches, and it would have enriched the film to have expanded on her politics. There are more wigs than Whigs, and if there's an Oscar hovering over this film, it'll land in the costume department. The clothes are divinely detailed, and with their curls and swirls, the towering wigs are virtual works of art.

The film's real power lies with Knightley. She easily could have gone over the emotional top, but instead gives a measured performance that increasingly draws you in. Knightley, who recently appeared in The Edge Of Love, has already grown as an actor since 2007's astonishing Atonement - a movie that belonged just as much to James McAvoy. The Duchess, however, is all hers.