The Bradford-born artist may have become the toast of the art world, but away from the occasional and inevitable champagne exhibition opening, he's never quite been part of the establishment.

He talks little about his private life, his work has none of the shock value of his successors and aside from regular outbursts about the terrible injustice of the smoking ban he has never courted publicity.

However, three years ago filmmaker Bruno Wollheim received a phone call. It was Hockney asking whether he would like to join him as he embarked on a mammoth project to paint the Yorkshire Wolds.

No one was more shocked than Wollheim. A year earlier he had been hoping to make a documentary of Hockney's life and work. His subject didn't hesitate in telling him what he thought of the plan.

Hockney wasn't interested in the past, he wasn't even particular concerned with what his legacy might be. He had turned 70 and as far as he was concerned whatever happened next was just a bit of a bonus.

By then, after 30 years in California, he had returned to Yorkshire with Jean-Pierre, his assistant and long-term companion. Living with his sister Margaret in Bridlington, he had begun to revisit the landscape of his childhood. However, while on one of his regular drives past the fields of East Yorkshire, spotting suitable scenes to paint, it dawned on him that he finally had the chance to win an argument
he had just sparked within the art world.

In 2006, Hockney had published a book called Secret Knowledge in which he claimed the grandmasters had used mirrors and lenses to help achieve many of their great works. He called it tracing. Others called it cheating and still more jumped to the defence of those long since dead, saying simply that Hockney was wrong.

Throughout his own career Hockney had used photographs, but having returned to the Wolds convinced that photography was an inferior genre, he was determined to go back to basics and prove that art in its purest form was the only way to capture the true beauty of a scene.

"I don't think he has any regrets about the book, in fact, David doesn't regret anything," says Wollheim.

"However, I think he was a little taken aback by the reaction to it, so much so that he pretty much stopped painting for two years. Some people in the art world thought he was trying to undermine 500 years of Western art history, that's a pretty major charge to lay at anyone's door."

With a camera following him, Hockney knew it would catch his idiosyncrasies, it might also tell a story of his own three-year journey, but he was certain that whatever the final result it would never be as great or as meaningful as the portfolio of work he produced along the way.

The jury remains out, but Wollheim's film, A Bigger Picture, which will be shown as part of BBC1's Imagine series next week, is much more than just an exercise in artistic one-upmanship.

The 60-minute documentary, edited down from more than 100 hours of footage, shows Hockney to be a softer, more mischievous and funny man than many previous profiles have suggested. It's the story of what happened after he moved back from California, but it's also the story of a man on a mission, of a man returning to his roots and recapturing his passion for painting.

"Sometimes I would get a call and David would say, 'Bruno get up here quick, you can't miss this'," says Wollheim, who made the journey from his home in London to East Yorkshire more times than he cares to remember.

"The seasons change so quickly that I had to be ready to go whenever he called.

"I never knew how long he would want me around or whether he would be in the mood for talking, but it was a real privilege to see him at work and I never forgot that.

"For a while I stayed at his house and at one point I did worry that perhaps I was becoming too embedded in his life and was maybe in danger of losing my sense of perspective, but I think it worked out okay in the end.

"Artists tend to be control freaks, but David didn't interfere. As far as he was concerned I was doing my own little work of art and he was doing his."

Hockney's move to America is well-documented. He likes to tell people he was brought up both in Bradford and Hollywood. There was a cinema at the end of the street and it was there, watching Laurel and Hardy films with his father, that he first noticed the one big difference between Yorkshire and California. The sun. Falling in love with the shadows it cast and liking the idea of a living somewhere overcoats weren't a wardrobe staple, it was inevitable that he would eventually leave England for the States.

His decision to swap his Santa Monica home for Bridlington was more surprising.

The move back, he admits, was gradual. The death of his friend and fellow Bradford artist Jonathan Silver in 1997 saw him spend much of that year in Yorkshire. In recent years, the only thing which had kept him going back to Los Angeles was his dogs and when they both died, he realised there was nothing to keep him there.

"I was surprised by how honest he was about how lonely he had become," says Wollheim.

"One day I think he just looked at his life out there and realised it was quite empty. It happens to a lot of people as they get older, the need to return to the familiar.

"I hesitate to use the word serenity, but I think David has found a kind of spiritual solace back in Yorkshire."

If it wasn't for his trademark white cap, Hockney could easily be mistaken for just another retiree walking along Brid's seafront, but when he gets out his canvases his talent remained undiminished. Ignoring a passerby who asks if he's free for a spot of decorating down the local pub and the group of horseriders who stop to admire his work, his rediscovery of the Wolds soon became an obsession.

Revisiting scenes again and again to capture first the changing seasons and then the morning mist or the afternoon sunlight, he grew to think of the fields and hedgerows as his "own garden". At one point he was producing a canvas a day and the last three years have been the most productive period in his entire career.

"I would be stood there looking at a muddy track, but David would be seeing a world of possibilities," says Wollheim.

"At one point in the film he says, 'In your head you can go anywhere, you'll never get there on the bus'. He's absolutely right."

During filming, Wollheim wasn't entirely sure his film had a beginning, a middle and until Hockney was given a 600 sq ft wall at the Royal Academy's Summer Exhibition he feared it might not even have an end. While some artists would have trawled through their back catalogue for a suitable selection of paintings, Hockney decided on a brand new work which would cover the entire space.

Bigger Trees Near Warter 2007 consisted of 50 different canvases. While he remained true to his new-found back to basics approach, when each canvas was complete a digital image of it was taken and on his computer he was able to build up a picture of the whole. Art and photography had come together to make one of the most impressive centrepieces of any Summer Exhibition.

"I did remind him of how he had once dismissed photography as wholly inferior to art," says Wollheim. "He pretended not to hear at first, but then with his familiar wry smile he quoted Walter Sickert and that was it. There's no point arguing with David, you know you'll always lose."

Imagine: David Hockney – A Bigger Picture, BBC1, Tuesday, 10.35pm.