Nostalgia for the Blitz has become one of Britain's most marketable commodities, especially now collective memory of the reality is starting to vanish.

Early in Lone Scherfig's??? Their Finest, I had the awful feeling that the heroine Catrin (Gemma Arterton), who works at the Ministry of Information, would wind up coining the phrase "Keep calm and carry on".

Thankfully, this does not occur. The story follows the making of a film within the film - an (imaginary) wartime propaganda piece inspired by a heroic rescue undertaken by two sisters during the Dunkirk evacuation.

Catrin, a talented writer, is drafted to help out with dialogue for women - or "slop", as her collaborators call it, flaunting the kind of misogyny that's joking only to a point.

These collaborators include the outwardly no-nonsense Tom Buckley, played by Sam Claflin, who with period spectacles, pencil moustache and Brylcreemed hair is less openly smouldering but much more winning than he was as the suave quadriplegic in Me Before You.

Inevitably, a romantic triangle takes shape, the third corner being a self-absorbed left-wing artist played by Jack Huston - who gives the impression of rehearsing to play the dying George Orwell, and might do so quite successfully when his face has a few more lines.

Adapted by screenwriter Gaby Chiappe??? from a novel by Lissa Evans, Their Finest is crowded with characters and character actors: Richard E. Grant as the stuffy head of the film division, Bill Nighy as a hasbeen matinee idol, Eddie Marsan??? as Nighy's agent, and so on.

There are times when the canvas seems overcrowded, but nearly everyone gets the chance to be both touching and funny. Inevitably, the best moments belong to Nighy, a happily shameless show-off who revels in his tailor-made part.

Their Finest is hokum, but it's honest hokum - which, like John Lee Hancock's underrated Saving Mr Banks, incorporates a defence of the simplifying, unifying value of popular entertainment.

In its modest, largely comic way, it paints an unusually convincing picture of how films are shaped by factors beyond the control of any one individual: the conventions of storytelling at a given place and time, the whims of a producer or director, the need to beef up the part of one actor or hide the weakness of another.

That said, the author of Their Finest itself is undoubtedly Scherfig, a Danish director who has now made several films in Britain - and who captures the spirit of the era rather more convincingly than Robert Zemeckis??? managed recently in his glossy thriller Allied.

True to the British convention of the stiff upper lip, Scherfig maintains a certain restraint in the tear-jerking scenes. Similarly, she portrays Catrin as a feminist by the period's standards without turning her into a 21st-century woman in disguise.

The Technicolor of the 1940s is convincingly simulated, and while there may not be the budget for panoramas of a bombed-out London, Sebastian Blenkov'???s often beautiful cinematography catches the foreboding feel of an English winter where light starts to fade in the middle of the afternoon.