Turner's Thames, BBC Four

Amid the splurge of programmes about London saturating the airwaves, apparently designed as a crude propaganda offensive to divert us from the impending Olympics clampdown, Matthew Collings's examination of the mystical relationship between the Thames and JMW Turner was thoughtful and rather touching.

Though a shrewd businessman who marketed himself skilfully enough to become wealthy in his twenties, Turner was an oddball and a bit of a loner, and indeed "an awkward cuss as a personality," as our guide put it. Collings himself comes over as at least mildly eccentric, stomping around snowy English landscapes in a thick overcoat looking, with his bushy beard and not-very-kempt hair, as if he spends his nights in Salvation Army refuges drinking soup. He delivers his commentary with a kind of pedantic chewiness, as if he were a rare species of critical ruminant.

However, what he told us about Turner's lifelong artistic connection to London's river seemed to winkle out some valuable nuggets of insight about the painter's methods and intentions, even if much of this sort of thing must, perforce, depend on hefty doses of speculation. Undaunted, Collings flung himself into the spirit of not only his programme but also of Turner's age, and the way his paintings were informed by the "cultural baggage of his times". What Turner did (he posited) was not just paint nature and landscape as he found them laid out in front of him, but cut, paste and transform them according to an inner mental framework.

It was Turner's acute sensitivity to the mystical properties of light which especially exercised Collings. "Turner makes light the vehicle of feeling," he suggested, slightly laboriously, and quoted the painter's reported death-bed comment that "the sun is God." Collings ventured up the Thames to Isleworth to survey the scenery that would have been fed through Turner's creative blender during the years when he used a boat as a floating studio, examining sunlight fragmenting off the water and the way birds could introduce shape and energy into a scene. Later, he retraced the painter's favourite journey down the Thames estuary to Margate. It was there that he created Yacht Approaching the Coast, a startling transformation of a commonplace nautical image into a blinding cosmic flash. Turner's Margate pictures contained "soaring abstract tremendousness," as Collings saw it.

Turner never really stopped being the slightly oikish son of a Covent Garden wig-maker, though he did turn his dad into his faithful studio assistant, but his vision continued to soar freely. His painting Rain, Steam and Speed presented the arrival of the steam train as a kind of demonic apparition, while behind the placid surfaces of the ever-popular Fighting Temeraire, Collings discerned "the magnitude and sorrow of loss", a post-Napoleonic Goodbye to All That. Turner's work is among the best known of any British painter's, but Collings's film still made you want to go back to it to look for things you'd missed.