Jeremy Northam provides many of the funny moments

The main characters are fictional - they and their story are based on Robert Harris's novel.

But the setting of the top secret Bletchley Park, where a rag bag assembly of brilliant minds worked deciphering German radio messages during World War II, was very real.

Enigma's characters are made to be part of that rag bag - there is flawed mathematical genius Tom (Dougray Scott), crossword champion Hester (Kate Winslet), femme fatale Claire (Saffron Burrows) and wonderfully camped-up intelligence officer Wigram (Jeremy Northam).

It does not fall into the common trap of feeling like it was made for a Sunday night on BBC Two

Despite the fact that Winslet is the only big name, the strength of the performances from the others means they deserve to become a lot better known, and soon.

In fact, Scott continues to be talked about as the next James Bond.

He certainly proved his ability to hold a leading role in a major film here, with an immersion in the character that makes the level of realism almost unnerving.

Tom is a Cambridge student who had cracked the code once before, had a nervous breakdown over Claire, and is sent home to recuperate.

Dougray Scott has been talked about as the next James Bond

But the Germans change their code and the Bletchley big wigs have little choice but to bring the prodigal Tom back - emotional instabilities and all.

Claire is the irresistible, manipulative siren, played with a deadly charisma by Saffron Burrows, who snares Tom and then becomes the centre of a mystery over whether there is a spy in the camp.

This detective story runs alongside the two love stories - first Tom and Claire, then Tom and Hester.

And the presence of the greater war thriller - where they are all (or almost all) trying to crack the German codes - runs throughout.

Confusion

The presence of so many threads provides lots of opportunities for tension, but it also becomes confusing, especially when they are all coming together at the end.

The whodunit is the dominant part of the story - but the film occasionally lives up to its name and leaves the viewer wondering what it is that has actually been done.

But at least it does not fall into the common trap among British films of feeling like it was made for a Sunday night slot on BBC Two.

On the whole, director Michael Apted - who was also behind James Bond's The World Is Not Enough - has struck a good balance.