Imagine: David Hockney, BBC1The Conspiracy Files: 7/7, BBC2

After decades in LA David Hockney returns to his roots and goes on a northern landscape-painting frenzy

With his owlish specs, straw-thatch hair, red bow tie and camp northern vowels, David Hockney was always going to be a national treasure.

On Imagine: David Hockney – A Bigger Picture, early footage from the 1974 film A Bigger Splash (rudely renamed "A Bugger's Pash" by Private Eye), about his failed love affair with the American model Peter Schlesinger, reminded viewers of his puckish charm.

Thirty years later, the television documentary-maker Bruno Wollheim finds him disillusioned with Los Angeles. His dogs had died, he told the camera; he felt alone and empty. It was time to return to his roots, to revitalise his art. For the next three years, Wollheim filmed the results of this homecoming, as Hockney discovered an activity he'd never tried before – painting in the open air. He began painting fields in watercolours, struggling to hold his canvas steady in the nippy gales around his native Yorkshire, and ended up painting in oils, eventually creating, in a mosaic of 50 canvases, a massive, shimmeringly beautiful landscape that filled one whole wall at the Royal Academy Summer Exhibition.

Hockney was excellent company, as he wrestled for the first time with art en plein air. In woolly hat and tramp's coat he resembled Compo from Last of the Summer Wine. His serious musings about art – how the Chinese deal in "the principle of moving focus", whereas all Western art presupposes you must remain dead still to paint or look at anything – were undercut by the reactions of others. As he painted a canvas by the roadside, a passing driver stopped to advise his assistant (in broad Yorkshire) "Got a bit of decorating in t'pub wants doin', if you can tell 'im".

As the project of painting a northern landscape through the seasons took hold, Hockney and his little entourage of bohemians moved into his mother's home in Bridlington, causing his down-to-earth sister Margaret (who shares his taste in granny specs) to flee. He flourished. He painted like a dervish. He realised he was, creatively speaking, on a roll at the age of 69. On the soundtrack, George Taylor's Elgarian cellos sawed excitedly. From unremarkable fields Hockney fashioned an Arcadia and watched it grow to epic proportions as the summer exhibition loomed near.

It was a sweet film that smoothly documented a late flowering of inspiration – a little too smoothly, as it shifted from Burbank to Bradford, from tentative daubing to triumphant artistry and walk-on plaudits from Damien Hirst and Nick Serota. Travel metaphors abounded, reminding us we were on A Journey. It was all a little too pat. You started to wonder: was Wollheim really a neutral observer of Hockney's life for three years? Or was he brought into the project shortly before the RA exhibition to fashion a documentary in hindsight?

That's called a conspiracy theory, and BBC2's The Conspiracy Files: 7/7 turned up a rich haul of paranoid bullshit around the events of 7 July 2005, when bombs went off on Tube trains and a bus in the heart of London, killing 52 people and injuring 784. The finger of blame pointed at four young British Muslims, who allegedly boarded a train together wearing rucksacks stuffed with explosives. That was the official story.

Through an hour of winks, nudges and hi-tech bleepings, the programme examined other dubious conjectures: that it was the work of the British Government, intended to demonise Muslims and persuade the British people to get behind the "war on terror"; that the four youths were framed; that the Tube bombs were pre-planted and the young Muslims duped into boarding the relevant carriages. (But how would you dupe them into sitting just where the bombs went off?) It was noticed that the train from which the bombers supposedly left Luton was running late that morning. Since, on the fateful morning, Benjamin Netanyahu was told by police not to attend a conference in King's Cross, was the whole thing an Israeli plot? (We later discovered that he was warned about a bomb 10 minutes after it went off.)

Coincidence prompted a number of conspiracies – about, for instance, Richard Jones, who left the bus just before it exploded in Tavistock Square and was discovered to have worked in an explosives factory – well, yeah, but as an electrician, back in the 1960s.

Among the fatuous theorisings was a genuinely sinister film called 7/7 Ripple Effect, a sophisticated compendium of conspiracies (with an annoyingly sleek voice-over) which was pressed on his congregation by a creepy and irresponsible cleric, Dr Mohammed Naseem (pictured left) of the Birmingham city mosque. Their readiness to take its idiot theories as gospel was genuinely alarming.

The BBC film itself didn't exactly help. So concerned was it with electronic trickery and raised-eyebrow voice-overs that, even when it was telling us that this or that conspiracy was false or unproven, it sounded more conspiratorial that ever. Its content was supposedly outrage at the falsity of witnesses and theorists, but its form seemed to endorse the culture of corrupted belief.