Meryl Streep is famously good at mimicking voices and accents, but she is also a genuinely great actress, and in The Iron Lady she gives a magisterial performance as Margaret Thatcher that ought to be a sure bet for an Oscar nomination.

It is largely because of her that the film makes for such gripping and often very moving watching. However, not even her brilliance can make up for the disjointed way director Phyllida Lloyd and screenwriter Abi Morgan combine a heartfelt film about grief and loss with a cruel depiction of dementia and an amateurish, old-fashioned biopic.

The film begins with Thatcher suffering from senile confusion in her retirement, and having imaginary conversations with her long-dead husband, Denis. It proceeds to tell the story of her life through a series of flashbacks. Many of these are prompted, unimaginatively, by photographs in her flat and accompanied by what looks like old news footage.

Besides the clunky structure, the film's greatest weakness is the unconvincing way it tries to use Jim Broadbent's ghostly Denis as a kind of Greek chorus. Broadbent has, for a while, been one of Britain's most overrated actors.

He gives such a lazy performance in The Iron Lady that you wonder at the director's competence. It is bad enough that he portrays Denis as such a wet blanket; it is worse that he makes little apparent effort to make him sound like the public school-educated, establishment figure he was. The result is all the more unsettling because Harry Lloyd, the impressive actor who portrays young Denis opposite the excellent Alexandra Roache as young Margaret, has bothered to get him right.

In another movie, with another star, the lack of verisimilitude in a key role might not matter. But given that the film's heart is the great central performance by Meryl Streep, who puts such effort into capturing perfectly Thatcher's mannerisms, voice and accent, the rest of the cast at least ought to try to be as believable and authentic as she is.

Perhaps the best of the supporting players is Anthony Head, who is perfect as the flabby, treacherous Geoffrey Howe. Richard E Grant as Michael Heseltine captures the egomaniac ambition and ruthless disloyalty but not the bluster.

If The Iron Lady is feeble as an effort to understand Thatcher's personality, it is surprisingly accurate as a chronicle of her career and premiership. Oddly though, the filmmakers seem unaware of the vital importance of Jewish advisers like Sir Keith Joseph and Alfred Sherman in shaping her views, preferring to stress her protégée-mentor relationship with the war-hero MP, Airey Neave, who was murdered by terrorists in 1979.

You also do not get a sense of the sexism that was so common in politics at the time that the Labour Party used the slogan "ditch the bitch" to campaign against Thatcher in 1982.

On the other hand, Lloyd and Morgan deserve credit for getting many key aspects of Thatcher's political style absolutely right.

There is a fine, difficult scene in which we witness just how rude she could be to her Cabinet colleagues. As shown here, her bullying was not just gratuitous but self-destructive.

Whatever prejudices one may bring to the film, it is often very moving. But, in the end, its depiction of grief and loss and the tribulation of dementia feels slightly cruel and exploitative, especially given that its subject is still alive.