He showed the book to his friend Andrew Stewart, a picture specialist then working for Phillips auctioneers in Leeds, who also ran a restoration business and gallery at his home in Harrogate. "This came on a very busy Saturday," Stewart explains. "I broke off to visit the bookshop, and bought the eight volumes that I could find there."

In his mind, as in Wood's, these were only the first of the hair's-breadth chances that led to an extraordinary discovery. "I was told they were part of a library of 3,000 volumes that had belonged to an eccentric and reclusive artist named Joash Woodrow," says Stewart. "He had grown incapable of caring for himself, had accidentally set fire to his house in Leeds, and been moved into sheltered accommodation." Stewart contacted those responsible for the house clearance, Woodrow's brother and sister-in-law, Saul and Sylvia, and on hearing there was a hoard of pictures and some wood sculptures remaining, arranged to meet them at the house.

It was a small, undistinguished post-war semidetached property on an estate at Chapel Allerton in north Leeds. "It was Sunday, which was Saul and Sylvia's regular visiting day — they had been doing this for years, driving 44 miles from Manchester. A skip waited outside. It was snowing. Saul and Sylvia were inside, cleaning the walls. There was little furniture to be disposed of. Much of what there had been — a television set, a piano, chairs and tables, even a window frame — he had turned into sculptures that had been mostly destroyed in the fire.

"Otherwise, I couldn't believe what I saw. The house was crammed with paintings and drawings, ground floor to attic, front to back, mostly saved from fire and water because they were so closely piled or stuck together. Paintings were stacked 50 deep. A 2ft-high pile of drawings in the kitchen contained at least a thousand works, their edges burnt and dirtied by fire. Sketchbooks were stacked on the windowsills. In the bare brick attic was a mural that had been painted by candlelight. Even in the overgrown garden were roof tiles and slates painted by Woodrow. He was an artist I'd never heard of. Nevertheless, with Saul's permission, I took 10 pieces home."

It was an intervention that was decisive for Saul. He admits that until then, he was dithering over whether to save his beloved brother's huge hoard or let them be taken off in the skip. "The pictures were hard to discern. They were darkened by Woodrow's tobacco smoke, then from the fire, and at first all appeared of the same colour, which was a dingy grey-blue," says Stewart. "I talked about them with my wife, Gillian, but we couldn't decide whether to get involved. 'Such things don't really happen,' we told ourselves, unable to believe what we had stumbled on. Are we about to invest in an 'outsider' artist, who probably produced the same image over and over again? My next thought was that this was an artist's life's work and it shouldn't be broken up before being fully assessed — therefore somewhere safe must be found to store it all."

So began the long process of storing the 700 paintings and 5,000 drawings, of cleaning and framing them. Using the skills of his assistants, Stewart is still a long way from completing the task. To date, 450 have been cleaned, while 50 have been assessed as beyond repair. "We at first picked out 40 pictures on which to begin the work of conservation, and each one turned out to be far more complex and rich than I had imagined. For a further opinion, we sent photographs of these to Nicholas Usherwood — the well-known critic, who happened to be a friend of Christopher Wood. It was weeks before I heard anything, and I assumed that he thought little of them."

In fact, they had gone to the wrong address. And here is another of the coincidences that appear to have saved Joash Woodrow's legacy. "The person who did receive them happened to be attending art classes, and she showed them to her tutor. Extraordinarily, the tutor turned out to be Nicholas Usherwood's wife, Jilly." Usherwood travelled to Harrogate and, amazed at the quality of the work, gave his support, spreading the word and writing a foreword to the catalogue for the first, small exhibition in 2002.

"On the day that the framed and cleaned pictures were stacked in the garden, I had a distinct sense that this was a historic moment," says Stewart. Pricing the pictures then at between £500 and £3,500, he recalls a "buzz of excitement" at the preview, though the exhibition was attended mainly by local people. All of them, probably without realising at the time, obtained investment bargains.

Within a year, Stewart was transformed from an employed picture specialist to a self-employed one and a full-time art dealer, championing Woodrow to a surprised art world beyond Harrogate. A handsome monograph has been produced, written by Usherwood and Wood, introduced by Stewart, with memoirs by Woodrow's brothers, Saul and Paul. There has been an exhibition in Leeds City Art Gallery, and more will follow this year. The highest price paid for a picture so far is £60,000.

It is not unknown for an artist to be overlooked during a lifetime, and to astound pundits and salerooms immediately after death. Van Gogh is the supreme example. It is rarer, if not unique, for an artist to spend a lifetime until the age of 73 obsessively creating a body of work, then to suddenly be discovered and celebrated as an important painter — while still alive, yet seemingly uninterested, in a nursing home.

Joash Woodrow's story is heart-rending and complex. He is neither a "primitive" nor an unsophisticated "outsider artist", as the Stewarts feared. He won a scholarship from Leeds to the Royal College of Art, London, from 1950 to 1953, and was a distinguished student along with Frank Auerbach, John Bratby and Peter Blake. Yet even at art school he was solitary: working hard, receiving commendations, but hardly remembered by his contemporaries. One exception is his friend at Leeds, Jack Coulthard. "We knew he was special," recalls Coulthard now. "I think he was the first really wise man we had met." He recalls that while the college climate in those days encouraged the murky naturalism of "the fashionable Euston Road school", Woodrow was already talking about Picasso. But a friend at the Royal College, Cyril Satorsky, recalls: "There was an intensity about him that closed him off from freely moving among other students and participating with any ease in social events even of a casual kind." The reclusive pattern of his life was developing.

In 1956, after college, he apparently had a nervous breakdown, after which he returned to the family's modest home in Leeds, and to the care of his strong-minded mother, Rebecca. She died in 1961. Of the family of 10, only Woodrow's brother Israel also still lived at home — Joash, it seems, especially after his mother's death, taking over most of the house for his paintings. Following Israel's death in 1986, he lived on alone, until 2000. There were no women and hardly any friends in his life. There were few connections with his scattered family, other than with Saul and Sylvia, who for years made their weekly visits to clean up Joash's environment, and put a chicken in the pot to vary Joash's frugal and functional diet.