If Helen Mirren's portrait of the Queen in Stephen Frears's film about the troubled aftermath of the death of Diana is one of her best turns, fully deserving of an Oscar nomination after her Best Actress award at Venice, the essence of this excellent film is Peter Morgan's screenplay.

How many British films have we seen that are better written than this? Not many. But then that's the basis of Frears's success. He works as much with his writers as with his actors and the result, if they are good enough, is generally a bit special.

The Queen is perhaps the most carefully balanced of his considerable oeuvre. In showing his (and Morgan's) version of what happened between the young Blair government and the royals in the days after Diana's death, the film doesn't take sides for long. It plays more than fair with both.

Who knows for certain what's true, what's half-truth or what's false? But what we do require from a film on this subject is a certain honesty, objectivity and feeling for the strange moment in time with which they are dealing. That is precisely what we get.

And it is certainly the basis of Mirren's surprisingly detailed and authentic-looking portrait of the Queen. It looks right, it sounds right and it has the whiff of truth about it.

Michael Sheen's Tony Blair is also distinctive, though a bit too puppyish at the beginning. I doubt that the Prime Minister was quite as naive when first meeting the Queen or quite as awed, since our monarch has had long practice at putting subjects at their ease.

It would have been easy to produce parody or satire, but though the film can be very funny - does the Duke really call the Queen "Cabbage"? - it also claims a seriousness that it triumphantly brings off.

Only the episode of Her Majesty and the Monarch of the Glen, whom she suddenly faces after her Land-Rover has broken down, looks forced and overly symbolic. And only the moment when she tells Mr Blair that everyone's popularity curdles and his will, too, seems like complete hindsight.

Otherwise, scarcely a foot is put wrong.The strong cast, including Helen McCrory as Cherie, James Cromwell as the Duke of Edinburgh and Sylvia Syms as the Queen Mother, could hardly be better. But, once again, it needs to be said that the writing is the thing, and a model of its kind, given a director who knows how to nourish it.

When you think of how awful this film could have been in more sentimental or blatant hands, you have to heave a sigh of relief and surrender to its interpretation of what, by any account, was an amazing piece of social and cultural history.