Dench takes up the story when the writer starts having problems

Iris splits the writer's story in two - the young Murdoch, in her mid 30s, is played by Kate Winslet, while Judi Dench gives a remarkable performance of the final years as she slips into oblivion.

Equally important is the role of John Bayley, the husband who was devoted to the sparky Iris when they first met, and still was - in a different way - when she was losing her mind.

The young John is played by Hugh Bonneville, with the old John played by Moulin Rouge star Jim Broadbent - and their performances are so in tune with each other that some film critics thought both parts were played by one person.

This film is a portrait of love - the young, exuberant love of the early days of a relationship that turns into the exasperated, devotional love of old age.

And in a style that is slow and English, it is a touching film that gets under the skin.

We follow Dench's Iris from the time that she first began to have problems, in her late 70s, with flashbacks to the 1950s Winslet era, which tells the story of how she and John met and how her career as a novelist began.

It also contrasts the two intellects - to remind us just how much the old Murdoch had fallen by the time the Alzheimer's had a grip on her.

At first, the only problems she has are things like repeating a sentence without realising it.

The couple try to shrug off the mental inconsistencies, but over time it becomes clear that something is wrong.

At one point, Murdoch dries up during a TV interview, blankly unable to remember what she had been talking about, and by the time she returns home she cannot remember where she had been or why.

When the postman delivers a copy of her latest, last novel, she does not even recognise it.

We are shown more aching examples of her decline as her condition gets gradually worse.

Director Richard Eyre gleaned many of the experiences by talking to John and reading his memoirs, but he also created some and borrowed some from his own experiences with his mother, who also suffered from Alzheimer's.

John said, after seeing the film, that he was "greatly relieved" that it had portrayed his wife fairly.

"Iris was being transformed, and very successfully transformed, into art - into art which I could enjoy as if I were almost a detached spectator, without feeling the pangs and the sorrows, the joys too," he wrote.

Hugh Bonneville plays the young John Bayley

He also praised Judi Dench for such a "wonderfully restrained and understanding portrait" of the troubled Iris - one that must make her front-runner for an Oscar.

Dench's portrayal is so convincing and transparent that it is very easy to forget that she is acting at all.

The writer's novels do not play a big part in the film, and it does not matter whether you had heard of her before or not.

It is the characters, the feelings and the nuances - every furrow of Dench's brow and every exasperated gasp from Broadbent - that make this film human and special.