Hattie, BBC Four

The way the BBC keeps knocking out these little biopics about the lives of various household names (John Lennon, Gracie Fields, Margot Fonteyn etc), you'd think there was nothing simpler than to get inside the mind of some complex public figure, deftly sketching in a bit of socio-historical background on the side with a bit of help from the props and archive department. And, as this low-rent effort to drill into the emotional life of the beloved comic actress Hattie Jacques amply demonstrated, you'd be completely wrong.

Throughout most of its 85 minutes I was poised on the edge of my seat, waiting for it to start. Screenwriter Stephen Russell had based his story on Andy Merriman's 2007 Jacques biography, or at least that section of it which dealt with her thoroughly improbable love affair with used-car dealer John Schofield. The relationship torpedoed her marriage to that other splendid British institution, John Le Mesurier. But while Russell had given us what I imagine must be the facts, I felt I understood less about the protagonists after I'd watched it than I had before.

The challenge of impersonating characters so firmly embedded in the national psyche is not to be squared up to lightly, and I wish this lot hadn't bothered. We know Ruth Jones is a gifted actress and writer, but about all she was able to convey in her portrayal of Jacques was that she was kindly, liked parties and cooking, and never fell out of love with Le Mesurier. Nowhere was there any inkling of the imperious, aircraft-carrier-like presence she was able to lend to the slenderest comic scenario. There were hints of self-consciousness about her weight, but nothing to explain why she would have spontaneously risked everything for a fling with a spivvy little ducker and diver driving a borrowed E-Type Jaguar.

There was, perhaps, supposed to be some attempt at Carry On-style humour in the way her caravan on the set of Carry On Cabby was seen rocking violently on its springs during her between-takes legovers with Schofield (oooh, Matron!). This was, at least, marginally more diverting than the tragic efforts to recreate Carry On scenes in black and white, which were so embarrassingly adrift from any spirit of the original that it was like watching Noël Coward translated into Korean. Elsewhere the script drifted hesitantly towards evocations of empathy or sympathy for Jacques and her emotional entanglements, which was a waste of time because her behaviour as depicted here was just stupidity of Olympian proportions.

Poor old Le Mesurier, as played by a slightly less than one-dimensional Robert Bathurst, was a man so self-effacing he wouldn't have cast a shadow on a sunny day. The characterisation seemed to have been based not on the inner Le Mesurier but on Sergeant Wilson from Dad's Army - the murmuring vagueness, the bottomless well of affability, the total absence of decisiveness, the innate ability to make women unable to resist mothering him.

Walking into his bedroom to find Hattie in an undignified bonking frenzy with Schofield, he averted his eyes in terrible embarrassment, and muttered, "Oh, I'm terribly sorry." Even when the malevolent, rat-like Schofield (an eminently kickable Aidan Turner) evicted him from his own bedroom and made him live in the sparsely appointed spare room upstairs, he was able to come out with lines like, "There's nowhere else I'd rather be."

What the hell did Jacques think she was doing? Why did Schofield fancy her? Why did Le Mesurier put up with it? And, after 85 minutes of this nonsense, who cared?