[Aboard the Hogwarts Express at Didcot, J K Rowling tells Elizabeth Grice, in her first post-publication interview, of her plans for Harry Potter, how she is adjusting to fame in 37 countries and what her daughter thinks of Quidditch]

SMALL wizards in pointy hats and trainers were being interviewed like celebrities on the platform at Didcot Railway Centre. The mothers of small wizards were being waylaid and debriefed about the spell that had been cast over their children by You-Know-Who. The fathers of the wizards pressed their noses to the opaque windows of the railway carriage, hoping to glimpse You-Know-Who's golden head inside.

And yet mysteriously, unbelievably, there are pockets of the country where the identity of You-Know-Who is not universally understood. Despite its starring role in the weekend's publishing extravaganza, Didcot, in Oxfordshire, is one of them. Some of the station staff had only an imperfect grasp of who the crowds of little people in starry cloaks were shuffling forward to meet. "We heard Harry Potter was coming," said a uniformed crowd controller. "But what's her name?"

J K Rowling, the sad-eyed woman at the centre of the publishing phenomenon of our times, does not mind this sort of confusion. Harry, after all, is her hero, too. She poured into this scruffy-haired orphan all the feelings of oddity and failure that she experienced herself as a child and gave him magic powers that he didn't know he had. Most of the day she looks as if she longs to jump on his broomstick, yell "Accio Firebolt!" and lift off over the heads of the crowds who are celebrating the much-hyped arrival of her fourth book, Harry Potter and the Goblet of Fire.

But, like her hero, she is not a quitter. She faces her fans with the steadfastness that Harry has when he faces the first task of the Triwizard Tournament. It is a test of stamina, of skill, of endurance. Last year, she played it differently. When Harry Potter and the Prisoner of Azkaban was launched, she tried to keep the lid on the hysteria and gave only one radio interview. It didn't work. The hysteria swept up to her firmly closed front door and seeped under the sill. People came with books to sign and questions to ask and when she didn't answer them, they called her a recluse.

"The bandwagon proceeded without me," she says. "I don't think anyone could say there wasn't hype then, so I think it's a moot point how much my participation blows the whole thing up. My decision this year was that I would go out and meet it rather than sit at home and be driven into the position of acting like a recluse because people are hammering on my door.

"An unprecedented five million copies of the latest book have been printed worldwide, in conditions of the greatest secrecy and security. The treasure trove is stashed in bright turquoise boxes with yellow seals and a warning - "unauthorised possession of this container may constitute an offence" -stamped on the side. The boxes were wheeled on to the station platform on trolleys, under guard, and remained under guard like unexploded bombs throughout the signing session.

"There are no words adequate to express my shock at what has happened," Rowling says, rather desperately. "I am amazed. Think of a stronger word and double it." She doesn't need to say this, because shock is in her face and in her voice and in the slightly abstracted way she allows herself to be shepherded from one publicity stunt to another.

After all the razzmatazz and unseemly adult scuffles earlier in the day at King's Cross - where the Hogwarts Express steam train carrying her on a four-day tour from London to Perth began its journey - Joanne Rowling arrived to a more orderly welcome at Didcot on Saturday.

She was colourfully underdressed for the conditions. The sky bulged with rain as she picked her way across the railway sleepers in high-heeled pink spangly sandals and a flimsy skirt. Didcot power station's cooling towers puffed away like a cauldron in Hogwart's Potions class.

Someone held an umbrella over her lightly clad shoulders, the way they do with royalty. Rowling is bigger than royalty, of course - but, like them, tends not to live up to expectations in the flesh. Onlookers want her to be more glamorous, less earthbound, perhaps more effusive. But her encounters with children are brisk and quite businesslike.

"I like children, but I am not sentimental about them," she says.

"She's just an ordinary person," says a boy with a Harry Potter lightning scar on his forehead and Potter spectacles painted round his eyes. "I'm here for Harry Potter."

Compared with recent lustrous studio photographs of her, Rowling is pale and slightly careworn. Ten hours a day working on the last chapters of HP4 have given her skin a writer's pallor. When she smiles for the camera, her lips do not open and her mouth goes down at the corners.

"The truth of the matter," she says, "is that I don't do as much of this kind of thing as I could. People have a tendency to forget that I am a single parent and I want to bring up my daughter myself.

"If I am going to be rocketing all over the world, spending two weeks in every country in which I am published [37 to date] and flying across the Atlantic every time someone wants to interview me, when am I ever going to see my daughter?"

Jessica is nearly seven now. Rowling vowed not to introduce her to the Potter books until she was seven, but all her school friends were talking about the latest exploits at Hogwarts School of Witchcraft and Wizardry.

"They would be asking her about Quidditch and she wouldn't have a clue. She was getting a lot of attention at school and it reached the point where I felt I was excluding her from a large part of my life, as well as from Potter conversations. So I broke my rule. Now she's totally obsessive about the stories.

"It's freaky because this is my daughter. But I think there is a big disconnect in her mind between me and the books. Her liking the books has made my life easier. Now, when I say: 'Jessie, I'll be ready in half an hour. I've got to finish writing this', at least she's looking forward to the outcome."

Rowling claims that despite the onset of fame and fortune, her life is really quite mundane. It is composed of the usual things: taking her daughter to school, answering mountains of letters with her PA, going out to a café to write, returning home to make the tea, putting off more writing, stopping to watch The Royle Family ("absolutely inspired") and trying to remember to bring in the rabbit and the guinea pig before she goes to bed.

But this pedestrian catalogue is deceptive. It disguises a seismic shift in the way she thinks now. "If you had met me a year ago," she admits, "I would still have been saying: 'My life has not changed'. That was wishful thinking. It was a control thing. Already I was wondering: 'What have I spawned?'"

Although it was getting so out of control, I liked to pretend that, at the heart, everything was normal, but it wasn't. I am slightly more honest with myself now."

For one thing, she had to admit that she was very rich indeed - possibly worth £15 million, though she never mentions figures - and needed sensible advice. She had to move home because her terraced house right on the street in Edinburgh was regularly besieged - though she denies being stalked.

"I bought that house before I had an inkling of what was coming. It was not so much scary as wearing. There comes a time when you want to shut the door at the end of the day and be with your family."

There was so much fan mail that the house was silting up, anyway: "I could have been my own full-time secretary." Now the American fan mail is diverted to and dealt with in America.

Another interesting change was that she could be extravagant, if she wished. And she could be confident - if she dared. One day, when she was feeling down about an untrue story that had appeared in a newspaper, she remembered having seen an aquamarine ring in a jeweller's window in Prince's Street."

I thought: 'Well, there is something good about your situation: you can go and buy that ring.' I have never in my life walked into a shop knowing that I was going to drop a lot of money, without even asking the price.

"That is a great way to be rich, it occurs to me in retrospect. It was the most extravagant thing I have ever done. No one really needs an enormous rock on their finger. It was self-indulgent and I loved it."

After the break-up of her short marriage to a Portuguese journalist (they were divorced in 1995), Rowling was quite poor. She fled to Edinburgh with her baby daughter and with the beginnings of Harry Potter in her suitcase. Depression over the death of her mother three years earlier was beginning to catch up with her.

She was living in a chilly flat. Her daughter was too small to qualify for state help with child-minding. So, in that most familiar part of the Rowling legend, she would traipse round friendly cafés, making a cappuccino stretch out for two hours while she wrote in the warmth.

She says her fictional soul- sucking creatures called The Dementors, who leech the happiness out of people, sprang out of her depression at this time. "They were out of my own experience: that absence of being able to envisage that you will ever be cheerful again. That deadened feeling, which is so different from feeling sad."

Within three years, she lost her mother, aged only 45, to multiple sclerosis, and gained a daughter. "They are the two things that have most affected me and my life - including Harry Potter. Nothing compares with them. He would be number three.

"It is no coincidence that Harry Potter's mother exerts a powerful influence through all the books. She has died protecting her son from the evil Lord Voldemort, and he is made aware that "to have been loved so deeply, even though the person who loved us is gone, will give us some protection for ever".

Rowling says she felt the truth of this the moment she saw her baby daughter. "From that day in 1993, I fully understood how inevitably and matter-of-factly you would protect your child. The first thing I thought when I looked down at her was that this was the person I would always put before myself. Not the big dramatic thing of laying down your life but putting your own considerations to one side."

She was "'very ready" at 27, to have a child. "I had done my partying, I had done my travelling. I had done my daredevil stuff and led a very independent life. I had lived fully as a childless person. It's as well to get some of those things out of the way before you have a baby because it's going to be a while before you get a chance again. But I never look at her and feel I have made profound sacrifices. She hasn't taken away; she has added."

For all Rowling's good fortune, being rich and carefree doesn't sound as natural a state as being anxious. "I remember vividly what it was like to be broke. Not a day goes by that I am not thankful that I don't need to worry about money any more. But it is unlikely that I will ever take it for granted. That is partly my personality and partly because when you have been broke, you don't go crazy. There seems to be an assumption that once you have money, you will want to revolutionise your life. I was happy with my life. I had great friends, a daughter I adored, I was doing work I loved. Those are pretty big things. I don't want to revolutionise my life: it is extremely nice.

"Apart from the obvious mustering of dark forces in Goblet of Fire - and there is a tremendous build-up of nastiness, slime, bloodshed and death - the thing you sense most powerfully under all the frenetic happenings, is anxiety. Potter is an anxious boy, an insecure wizard as well as a brave one, the target of bullies and plotters.

Although Rowling says she was never bullied "with a capital B" at her school near Bristol, she often felt a misfit. "I sounded very south-east, as opposed to south-west. I was a bit too clever for my own good. I understand what it is like to feel a complete fool, to be very confused, to be rubbish at sports, the last person to be picked for a team. I'd hear a groan go up because they had to have me."Harry excels - and he doesn't. He's really flying by the seat of his pants. He gets by with a combination of nerve and luck, which is fairly autobiographical."

Rowling says she would love another child but thinks it's "highly unlikely to happen". There isn't a man in her life except for Harry. She is so used to him and his fellow characters that she doesn't have to think herself back into the stories. "It's like picking up the phone to someone you know. They feel like friends. I can drop in on them. It is going to break my heart to say goodbye to them. It really will feel like a bereavement. But there are aspects I won't miss at all.

"Will there be more books? "I will carry on writing, to be sure. But I don't know if I would want to publish again."