You can tell that John Matthews loves books. They crowd the living room, dominate the study and are piled up by the stairs of his house in Headington. He has written around 50 of them himself, although he's not sure of the exact number. "I've lost count," he said.

His books range widely over Celtic and Arthurian themes. John's first published book was The Grail: A Quest for the Eternal, which has never been out of print and he has just finished another book on that topic. "I've come full circle, 23 years on, to actually reassess the whole idea of the Grail and what it means to me and people in general," he said.

His interest in Arthurian and Celtic topics has grown throughout his life, since he discovered the legends while in his teens. He said: "The thing that was strongest in my childhood was a love of fairy-tale and folklore."

By the time he was ten, he wanted to be a knight. "I discovered I couldn't do that, but the next best thing was reading about it and then I discovered the Arthurian legends when I was about 15."

By this time, John knew he wanted to be a writer, but did not foresee how Arthur would lead to everything else. "When you start researching," he said, "You start finding that the earliest levels are not medieval knights in shining armour, but these Celtic warriors. I began to explore that -- and that led me to Celtic traditions and beliefs and the ideas behind them."

John's extensive knowledge, garnered over 30 years, led to a position as historical advisor on last summer's blockbuster King Arthur, which came out on DVD on November 29. John was a little dubious at first at the idea of getting involved with the film industry. "I thought, this is Hollywood, they're not going to want to listen to anything I say or they'll listen and then dismiss it. But much to my surprise, when I arrived on set and started pointing out things that were wrong, they wanted to change them and did."

What John particularly liked about the movie was that the writer, producer and director wanted to take the Arthur story back to its most likely origins. There is a known historical figure, Lucius Artorius Castus, a nomadic warrior from Eastern Europe, a Sarmatian, who became a Roman officer stationed on Hadrian's wall. "Where it gets really exciting is when you discover that a lot of the stories told by the Sarmatian people are exactly the same as or very close to some of the Arthurian stories," John said.

Although it is highly questionable whether Arthur ever actually existed, Castus is the most plausible candidate, according to John. As for Merlin, John believes that the historical fragments that are left point to his having lived on the borders of Scotland.

"Myrddyn was a late sixth-century prince who fought in a great battle that we know did take place, the battle of Arderydd, where he apparently went mad after seeing some of his close kinspeople killed in battle and is supposed to have run off into the wood of Caledon. He wandered for a time in the forest, eating roots and berries and talking to animals."

John believes that Myrddyn became anglicised as Merlin, before elements of his story were knitted into Arthur's. He traces the roots and impact of both Merlin and Arthur in two books that were published when the film came out. Merlin: Shaman, Prophet, Magician and King Arthur: Dark Age Warrior and Mythic Hero, which both include stills from the film, have already gone into reprint because they have sold so well. Indeed, the film has ignited John's career, since before it came out, his books normally catered for a niche market. He is finishing The Grail: A Secret History, a look at the long story of the Grail from its beginnings in ancient history to the present renewal of interest, promoted by such fictional retellings as The Da Vinci Code.

He is planning a book on Castus and another on the history of pirates which will coincide with the sequel to The Pirates of the Caribbean. He also has feelers out to advise on more films.

John moved to Oxford from London 17 years ago with his wife and fellow writer Caitlin and son Emrys. "We wanted somewhere that was convenient for London and airports, but most of all we loved the city," he said.

"It's just so beautiful. It's a medieval city and we're both medievalists, so what better place could you be."

King Arthur: Dark Age Warrior and Mythic Hero is published by Carlton at £14.99.