THERE IS A MOMENT in The Edge of Love, the new British film - I was going to say about Dylan Thomas, but I'll hang fire on that for a moment - when the bohemian Welsh bard explains why he needs to sleep with so many women. “It's because,” he says, “I'm a poet, and a poet feeds off life!” I've seen this line quoted twice now in reviews to illustrate the awfulness of The Edge of Love's script, but in fact it's an unfair example: the delivery of it by Matthew Rhys, the actor playing Thomas, signals quite clearly that the poet is being ironic, aware of the cliché.

But the need for irony at this moment in the film tells us something more problematic about The Edge of Love, and indeed about the whole genre of heritage movies about great literary figures. This is about the only point in the film where Thomas refers to himself as a poet, and he's not allowed to do it seriously. Similarly, although he's seen scribbling away, and there are bits and pieces of his work recited (very accurately: it's a brilliant vocal impression by Rhys) at no point do we have a sense of Dylan Thomas the poet.

You could watch The Edge of Love many times and still not gain a single ounce of greater insight into Fern Hill or Do Not Go Gentle Into That Good Night or indeed Among Those Killed in the Dawn Raid Was a Man Aged a Hundred even though we are shown Thomas composing that poem. The showing of this, in fact, is a case in point: the scene has Thomas trying it out on his wife, Caitlin, in bed, and the meaning or quality of the poem is irrelevant. It just becomes another device through which to enact the sexual drama.

Although they keep on getting made, I'm not sure I've ever seen a great film about a great writer. The cinema is not entirely comfortable with the idea that the written word might be extremely powerful. It finds it easier to express the work of visual artists, as John Maybury, The Edge of Love's director, did very successfully in his previous film about Francis Bacon, Love is the Devil.

But when it comes to writers, these movies always do the same thing. A couple of weeks ago I argued that representations of Jane Austen tend to obsess around the reductive idea that her need to write was something to do with her haplessness in love, and of course it is love, rather than art, that scriptwriters will continually look to explain away great writers. Tom and Viv: it's there in the title; look no farther for an understanding of T.S.Eliot than his marriage. Wilde: nothing about Oscar the novelist or dramatist, everything about Oscar the tortured lover of Bosie.

Shakespeare in Love: need I say more? (Incidentally, Love is the Devil does also suggest that the key thing about Francis Bacon was his love life).

Sylvia, the 2003 film about Plath, is the most heinous example since the Sylvia Plath estate refused permission to use her poetry in the movie. I wonder if there was a moment when the producers had a meeting about this and someone actually said: “Well, what does it matter? No one gives a toss about the poetry anyway: all they care about was that her and Ted had a terrible marriage.” Which may be true, but then why do they still need to call the movie Sylvia? Why not just make a movie about a terrible marriage?

Similarly, if you're not going to say anything about his poetry, why bother to make a movie about - or at least, centred around - Dylan Thomas? Without his poetry, Thomas was just another selfish alcoholic boor. The Edge of Love is really just another love triangle movie, and interesting though love triangles always are, what exactly is gained by placing Dylan Thomas at the tip of that triangle? The answer - apart from a grant from the UK Film Council - is instant gravitas. But instant gravitas is a bit like instant coffee: only the tasteless cannot immediately tell it apart from the real thing.

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