The film also has the odd ingratiating anachronism. Early on, when the Duke complains about the intricacies of women’s clothing as he undresses his new bride, she protests: “It’s just our way of expressing ourselves.” No 18th-century woman, from dairymaid to duchess, could have said such a thing. Besides, it rather goes against the whole dynamic of the story, which is of a strong woman defying the rigid conventions that were, if anything, symbolised by the almost ­prison-like structure of their clothing.

While my companion at the screening flinched at this kind of thing, I found that I could live with it. Because, taken as a whole, this is a grand, glamorous and unapologetic film. Forced echoes of and allusions to the present notwithstanding, it has absolute confidence that its story is one worth telling. The characters are well written, and even if you pretty much know what’s going to happen to them, you find yourself caring about how. And I, for one, am not above a bit of excitement at learning the odd new fact: I had no idea, for example, that Sheridan’s The School for Scandal drew on the Devonshires’ marriage.

Put simply, it makes the past look beautiful, like a place you would be happy to visit. The director, Saul Dibb, who made the TV adaptation of Alan Hollinghurst’s Line of Beauty, has eschewed the ­bleached-out, rather threadbare look of some recent costume dramas and given his film the full-­blooded treatment. It is at once sensual and majestic. The Naval College at Greenwich is again pressed into service for London street scenes, but the film’s particular sense of confidence and opulence is conveyed by the use of some of this country’s finest houses and urban architecture. Chatsworth, Holkham Hall, Kedleston Hall and Bath’s Royal Crescent should all see a rise in visitors after this film.

I’ve said before in this column that Knightley is not one of my favourite actresses; she is too model-like for me, and there is that strange, jagged grin. Judging by the portraits of Georgiana by Gainsborough and Reynolds, she also lacks the Duchess’s curves. But she carries it off. As her duchess sweeps through grand entrances and up marble stairs, it never looks for a moment as if she shouldn’t be there.

This is, I think, the best thing Knightley has done. Her name means a lot to the readers of Heat magazine, and it will draw them into the cinema. Some of them, maybe just a few, might find that their interest in the past is pricked enough to look further. If so, she would also have performed a small but valuable service.