British period pieces are regularly mocked as pretty pieces of cinema devoid of real bite. But you couldn’t call Stephen Poliakoff’s first film for more than a decade namby-pamby.

It is, in fact, a thriller set among the aristos still ruling the roost at the time of Chamberlain’s attempt to placate Hitler — and it spills a lot of bloody bile.

A splendid cast — including Bill Nighy as the Tory MP and family scion, Jenny Agutter as his timid wife and Julie Christie as Aunt Elizabeth — inhabit the equally splendid setting of Walsingham Abbey in Norfolk, cavorting merrily as war approaches.

They support Chamberlain while others, less likely to lose everything by any impending conflict, support the upstart Churchill.

Poliakoff knows his history and rightly accuses this gilded lot of machinations tantamount to treason in their attempt to forestall the end of what was for them a golden age.

It was not for nothing that my own well-born mother strode around London with a placard reading “Hands Off Hitler”. My father simply went on hunting.

The trouble with the film is that the politics become increasingly drowned out by the thriller aspect of the story; in the end, unlikely melodrama makes Poliakoff’s argument seem weaker than it is.

Romola Garai plays the upstanding family’s adopted daughter, a budding actress in love with a Churchill-supporting Foreign Office man (Charlie Cox) who discovers a dastardly plot to destabilise the Churchillians and is made to rue the day by those she thought loved her.

When I say the cast also includes Hugh Bonneville, Christopher Lee, Jeremy Northam and Corin Redgrave, you would expect fireworks — but it doesn’t work out quite like that.

Nighy in particular gives a curiously detached performance and the others are not always able to make a rather ordinary Poliakoff screenplay sing.

There’s much to admire in the film, such as sequences of the often forgotten panic when family pets were systematically put down as war became inevitable, but there’s quite a lot to argue about too.