Costume dramas are very much Keira Knightley's forte, as exemplified by Pride and Prejudice and Atonement. (King Arthur and Silk can safely be ignored).

And she triumphantly proves the point again with her excellent performance - for my money, far and away the best she has ever given on screen - in this persuasive true story of the 18th-century beauty and political activist Georgiana Spencer.

Georgiana married the Duke of Devonshire (played by Ralph Fiennes) with a marriage contract that paid handsomely for the birth of a son and heir and ended up living in a ménage à trois with the duke's mistress and her best friend Lady Bess Foster (Hayley Atwell).

Given the storyline and the fact that Georgiana was an ancestor of Princess Diana, parallels between the film and real-life events ("there are three of us in this marriage") are inevitable and, indeed, feature in advertising for The Duchess.

The film stands up even without the Diana connection, in large part thanks to Knightley, who looks stunning, as usual, in period costume and vividly brings Georgiana to life. Fiennes too puts in a compelling performance, brings depth to his buttoned-up and heartless character. The pair are aided by a well-cast gallery of supporting players including Atwell and Charlotte Rampling - who suddenly seems to be everywhere these days - as Georgiana's marriage-broking mother.

Co-writer Saul Dibbs's direction makes much of his, Jeffrey Hatcher's and Anders Thomas Jensen's adaptation of Amanda Foreman's best-selling biography. Fine cinematography (Gyula Pados), Michael Carlin's beautiful production design and immaculate costumes (Michael O'Connor) all help to deliver a gorgeous drama whose good looks should not be held against it.