Maybe so, but the vivid descriptions of the various matches which take place at Hogwarts make it sound more like combat. Also it is something which Harry is good at. "It helps him belong," and Rowling agrees that central among the several prevailing themes of the books is Harry's need to belong.

There is nothing soft or delusionary about the story. The prose is highly descriptive but the language is flat, neutral, as accentless as she is. But the tone is English. Harry's saga is funny but it is also black, violent and even threatening. "Children appreciate real life." Harry is the victim of multiple miscarriages of justice. "I think there is a strong sense of powerlessness. That's one of the big things about being a child, you have no power." For all its fun, Hogwarts is very strict and competitive. "I think children respond to the idea of rules because they know they are there to be broken and also there is the fact that the world is a tough place."

All kinds of claims for the books have been made. She has even been unconvincingly compared with Jane Austen and Henry James, but Rowling's success is about the enduring appeal of story. "That's it exactly. I planned the entire story years ago. I already know how it ends. It just became several books instead of one. But it is one story, Harry's story."

Another comparison is with the sevenbook Chronicles of Narnia by C. S. Lewis, which have a moral and a strong religious theme. Still, standing head and shoulders above so much of this writing is the achievement of J. R. R. Tolkien. "I do admire him," she says of a writer whose obsessive attention to detail exceeds even her own.

Her uses of motifs and devices is similar to Tolkien's. Her books evoke a society within a society and a fantasy world of magic superimposed upon everyday life. Tolkien creates a fantastical world possessing a complex culture, history and several languages. In The Hobbit, Bilbo finds a ring which makes him invisible; Harry has the Invisibility Cloak. For Tolkien, the ring becomes a symbol; in Rowling, the cloak is a good comic device. "Cloaks are more fun than rings, you can trip on them, tear them, they can fall of - they are fun."

She has a sister. "It was a small family, maybe that's why I fell into making up stories. I played with boys who were good pals, platonic friends. But I always needed time to be on my own. I still do. I like sitting down to write, to create my own world."

Where did Harry come from? "He just happened. I was thinking about him for ages. And he had to be a boy. It would have been a big mistake to have made him into some kind of feisty, football-playing tomboy." He is also no saint. "He wants to do well, to find himself. He is brave and he also tells lies." Rowling has never allowed her readers to dictate what she writes. "I like my readers, I love entertaining them. But I write these books for myself. I don't want to find myself thinking, `oh dear, I'd better not write that in case it annoys my readers'."

For all the mass adoration, there has been criticism. Some parents objected to the episode in which Harry and Ron, having failed to get on the school train back to Hogwarts, decide to get there courtesy of Ron's dad's flying car, the famous blue Ford Anglia. "They are not supposed to take it. But the plan appeals to Ron's recklessness." For Harry, the school, with its interesting resident ghosts such as Nearly Headless Nick and Moaning Myrtle and the dreaded Privet Drive, represents sanctuary. "They steal the car and they are punished. They crash it." The books have a subtle morality. They are about good and evil and human nature in general.

It all began about 1990. Her mother died. "She was 45, she was only 20 when she had me." That loss has never left her. It never will. Just as Harry constantly hears his dying mother's words. "It's no coincidence that that's so much part of the book." Rowling was also trying to write a novel. She found herself moving between the novel and Harry "and Harry began to take over. He won".

At the University of Exeter, she studied French and Classics and on graduating, worked for Amnesty International, before setting off to travel. This led her to Portugal, where she met and married a journalist. "It ended very quickly. When Jessica [her daughter] was three-and-a-half months old, I left and came back to England." It was 1993. Although Rowling makes a point of saying she is extremely impractical, she does strike one as very practical. Or perhaps it is a practicality learnt from hardship. After some time in Manchester and London, she decided to move to Edinburgh where her sister lives. "I felt Jessica could have a better quality of life living in Scotland with no money that we would have in London with no money."

Her situation was to become very difficult before it began to improve. Most of all, she had to deal with her own anger and sense of injustice. She got a typing job and a teaching qualification. Within a year, she was teaching French but began to realise she wanted to work part-time in order to write. By the mid 1990s, she had "pages and pages" about Harry. Much has been made of the decision to use her initials instead of name in order to attract boy readers. Again she laughs. "It was the publisher's idea, they could have called me Enid Snodgrass. I just wanted it published." It was and she was happy enough with an advance of £2,500, aware that many children's writers don't get royalties. Harry Potter and the Philosopher's Stone appeared in 1997 to good reviews and a Smarties Gold Award.

Harry Potter and the Chamber of Secrets appeared the following year with more prizes and even louder praise. A cult was developing. As for Harry Potter and the Prisoner of Azkaban, which won the 1999 Whitbread Book of the Year, it merely consolidated what has become a phenomenon and set the scene for recent weeks.

The fourth book was originally due for delivery to her publishers last Christmas, but Rowling missed her deadline by two months. "I had problems. This has been the hardest one to write. I was halfway through it, or at least I thought I was halfway, and I realised I had a great gaping hole in the plot. It had never happened before. Suddenly my plan failed me. And I had to unpick the whole thing."

She writes in longhand and then types the first of several drafts. Her characters are people she knows very well. Another of the advantages of bringing a story through several volumes is seeing characters develop. She enjoys writing the grotesques as much as the good guys and as for writing the comedy, she says: "I don't think you can really make yourself laugh, but you can amuse yourself. I do. The new one made me cry. It is a very important one, it marks the end of one phase." How would she describe her humour? "It's very black, even quite sick."

A couple of times, she makes the point she is not contracted to any one for the seven books. "I know I don't need to write another word, but I will." In three books' time, Harry Potter will be 17 and ready to leave school and start his own life. This reality has the same mixture of satisfaction and sadness of watching a child grow up.

Rowling seems to have achieved the impossible, encouraging non-readers to read and despite the hype, is remaining a realist. As the elderly Edinburgh cab driver said on the way to the airport: "She's a sensible lass. The fuss won't turn her head. I've read two of them".

Harry Potter and the Goblet of Fire is published by Bloomsbury, £14.99 in UK

It is reviewed by Niall MacMonagle on Saturday's books pages