After a while, I lost count of the clichés in Shirley (29 September, 9pm). A girl who stuns a reluctant booking agent in an audition, his mouth falling wide open in amazement? Tick. A girl - she is a little older now - who can quieten even a fractious Glasgow audience by belting out "Stormy Weather"? Tick again. A girl - actually, she is a woman by this point, draped in diamonds, sequins and fur - who refers to herself in the third person every time she stands in front of a dressing-room mirror? Tick, tick.

Would there, I wondered, be scenes with paparazzi, involving enormous, old-fashioned flashbulbs and pork-pie hats? Of course there would. Also, in no particular order: ancient newspaper headlines ("Shirley: the new sensation!"), pouty tantrums and a secretly gay husband (naturally, said husband carried his new bride over the threshold of the swanky pad in Maida Vale he'd purchased by way of a wedding present). Wow. And the wonder of it is, the whole thing lasted only 70 minutes.

In the end, though, none of these things got in the way of one's enjoyment. You can have the ropiest script in the world and the smallest budget (I think this one's must have been about 45 quid - our heroine wore the same evening gown twice), but if the performance at the heart of a biopic is truly sensational, it won't matter a fig.

I don't think I've seen Ruth Negga on television before (she is best known for her role in Misfits, a show I've never dared to watch for fear of discovering just how uncool my jeans are). But, as Shirley Bassey, she - let's throw in yet another showbiz cliché here - knocked me dead. She had the accent just right (Bassey grew up in unlovely Splott, Cardiff), and the manner: shyness becoming confidence and then, as the money rolled in, out-and-out ghastliness - at which point, the accent went from Splott to something more affected, a sort of Welsh
Hyacinth Bucket.

She could do that Bassey thing with her hands, halfway between a wave and a flick. Even her lip-syncing wasn't the usual embarrassment. It was spectacular to see and I would love to know if Dame Shirley watched it on her widescreen at home in Monte Carlo. By all accounts, it takes quite a lot to discomfort Miss Bassey. But this, had she been able to stand it, would surely have done it. Negga's performance felt somehow truthful, as if she had seen right into her.

Two things helped her. First, an excellent supporting cast: Lesley Sharp as her tough-as-nails mother, Eliza; Charlie Creed-Miles as her stalwart manager, Mike Sullivan; and Henry Lloyd-Hughes as her slippery first husband, Kenneth Hume (according to this version of events, the beginning of the end of the marriage came when Shirley caught him mooning over his wan and gingery chauffeur).

Second, Negga was not required to age (the film ended in about 1964, when Bassey was in her thirties). Prosthetic wrinkles are so rarely the actor's friend. Whatever else I think of Shelagh Stephenson's script, it was an excellent idea to cover only the early years, this period having been given fresh heft by the discoveries of Bassey's most recent biographer, John L Williams.

It was Williams who last year revealed that Bassey's father, a stoker, was sent to prison for unlawful sex with an underage girl, after which he was deported to Nigeria (Shirley was then a toddler). And it was Williams who confirmed that the singer's mother had ten children by four different men, two of whom, as little girls, she abandoned in Hartlepool shortly before she headed to the wilds of Tiger Bay.

The latter revelation is important because it explains, at least in part, why her mother was determined for Shirley to leave her own baby daughter - born when she was 17 - in the care of her sister while she pursed fame and fortune. Eliza, as pragmatic as she was feckless, had abandoned two girls, for nothing. So why wouldn't Shirley leave one, especially if there was a mink stole in it?

And this, I suppose, was where the film succeeded, clichés and all. If there is a chip of ice in Dame Shirley's heart, someone put it there long, long ago.