Presumably hoping to tempt hoardes of Princess Diana fans to the cinema, the publicity campaign for Keira Knightley’s new period drama The Duchess has cynically focused on the parallels between the late ‘People’s Princess’ and her direct descendant Georgiana, the Duchess of Devonshire.

In fairness there are some striking similarities between Diana and her distant relative. Both married young and were trapped in loveless marriages in which there was a third party involved. Both struggled to find happiness but were adored by the public.

But the duchess' story is gripping enough without the Diana comparisons.

Naive 16-year-old Georgiana (Knightley) is paired off with the considerably older Duke of Devonshire (Ralph Fiennes) with the blessing of her mother who considers him an excellent catch and persuades her daughter he is in love with her.

But Georgiana quickly realises he has agreed to the marriage solely on the basis she produce an heir and he continues his lifestyle of a single man with a string of affairs.

Realising her husband has no interest in her she throws herself into society events and quickly becomes the darling of the 18th century social scene with her pioneering glamour.

But behind the glitz lies a terrible unhappiness. After the birth of her second daughter the duke seduces Georgiana's only friend Lady Elizabeth Foster (Hayley Atwell) and the pair are soon living together under her nose in the family home.

Fiennes puts in another brilliant performance as the emotionally inept duke who is so consumed by his desire for a son that he neglects both his wife and daughters.

Rather than portraying him as an out-and-out monster, Fiennes cleverly offers glimpses of the unhappy man beneath the frosty exterior.

The most revealing moment comes in one of the final scenes when watching his daughters play happily in the garden he comments “how wonderful to feel that free”.

Putting in possibly her best performance to date, Knightley manages to be both vulnerable and feisty as the tormented Georgiana.

You can’t help but inwardly cheer her on when she finally acts on her feelings for politician Charles Grey (Dominic Cooper), whose campaign she has backed, and she briefly experiences love at last.

Georgiana falls pregnant with Grey’s baby but is forced to give her up in order to keep her other daughters as the duke will stop at nothing to protect his reputation.

Knightley brilliantly portrays the emotion of the heart-wrenching situation as well as establishing a convincing rapport with on-screen lover Cooper.

The Duchess offers a fascinating glimpse into the private life of a remarkable public figure - well worth a trip to the cinema.