It's a busy day at King's Cross station: Muggles are clustering round the steam engine at platform 9 3/4. Yes - it's the Hogwarts Express - the very train that takes pupils to Hogwarts School of Witchcraft and Wizardry in Harry Potter and the Philosopher's Stone. It's there to celebrate the paperback edition of the sequel, Harry Potter and the Chamber of Secrets, and the platform is seething with wannabee wizards hoping to meet the author JK Rowling.

She's had a busy week of award ceremonies, bookshop events, readings, TV appearances and school sessions. I talk to her on the train as she heads back home to Edinburgh. Isn't she exhausted by all those children?

"Oh no! - as an ex-teacher it's just so liberating to go in front of a class simply to entertain them. And it's great when they've read the second book - they know their Slytherins from their Gryffyndors - and they're familiar with the rules of Quidditch."

JK (Jo) Rowling is every bit as witty and subversive as you'd expect: hip and glamorous too, in a long black velvet coat. But she doesn't much like interviews. In 1997 she hit the headlines with the huge success of her first Harry Potter book. "But the publicity knocked me sideways.

I was portrayed as a penniless, divorced, single mother, pushing a buggy round Edinburgh, on income support, and writing in play parks and cafés while the baby slept. Yes, of course I realise it was good copy - and yes, it was true; but I resented the reporter's implication that I should have gone out and got a proper job instead of sitting around writing. It wasn't a soft option. Not many people would have put up with those conditions. And then, just as I was able to feel secure for the very first time since my daughter was born, I was defined by what was in many ways the saddest part of my life. It was hard enough at the time, without having to relive it. It was dreadful - for a long time it stopped me writing. But don't get me started on that."

She laughs - and obligingly skims through a career that includes comprehensive school in the Forest of Dean; Exeter University ("fantastic, if not quite the chance to be the 'radical' I planned"); two years at Amnesty International (researching human rights in Francophone Africa); a TEFL course in Manchester; a teaching job in Portugal (and a marriage that didn't work out); after which she came to Edinbugh to be near her sister, and where she eventually did a PGCE and became a school teacher. "But I'd always wanted to be a writer, and wherever I was, whatever job I had, I was always, always writing like crazy, and when I came back from Portugal, I had not only a four-month-old baby but also one third of Harry Potter. So I had to finish it."

Times were hard, accommodation was grotty, but she doesn't dwell on that, she talks instead about her old friend Sean who lent her money to put a deposit on a flat, and she raises a glass to the Scottish Arts Council whose bursary finally enabled her to get proper childcare so she could finish the second book. Several big publishers had turned Harry Potter down as being "too long", "too slow" or "too literary", but Bloomsbury took it on in 1996. And then the following year it was bought by Scholastic in the US. She makes me laugh describing the night her agent rang from New York to say that an auction was taking place: "An auction? I thought . . . Sotheby's Christies? Antiques? What IS he on about? Then I realised, it was my book. That was amazing. I was like Elizabeth Bennett, you know, 'She knew, rather than felt she was happy'."

She's thrilled, if a little bemused with the latest developments - there's a film in the offing, she has won top book awards, and sales have now passed the half-million mark. How does she account for this? "I suppose it's mainly word of mouth," she says rather vaguely. "I think children just tell one another about it."

With its galloping storyline, awesome abundance of magic, capricious humour and a wicked eye for detail, Rowling's rich mix of bravery, friendship and adventure centres on two classic ingredients of children's fiction - the hard-done-by orphan and a boarding school.

"Of course it's been done before," she says, "but Harry HAD to be an orphan - so that he's a free agent, with no fear of letting down his parents, disappointing them? and Hogwarts HAS to be a boarding school - half the important stuff happens at night! Then there's the security. Having a child of my own reinforces my belief that children above all want security, and that's what Hogwarts offers Harry." She does concede, on reflection, that with filthy dungeons, haunted toilets and murderous intruders, the security aspect of Hogwarts is perhaps "not immediately apparent" but, she adds briskly, "there are some very strict rules".

Successful sequels are notoriously difficult to achieve but the second book is even better than the first. There are seven books planned and Rowling has already written the final chapter of the last book in advance. She hints at unexpected twists ahead as Harry, the young wizard, grows up: "And he does grow up - in book four the hormones are going to kick in - I don't want him stuck in a state of permanent pre-pubescence like poor Julian in the Famous Five! And the struggle between good and evil will intensify; there will be deaths - I've thought long and hard about this - I'm well aware of younger readers, but evil is not something you can deal with lightly - there are consequences, there are victims. Children always ask if Harry will get his parents back, but some things, even in the magical world, are irreversible."

Outlining these complexities, she speaks of some of her own beliefs and inspirations - including her involvement with the Church of Scotland, and in particular, her long-term admiration for the wildly uncoventional author Jessica Mitford - who famously ran off to the Spanish Civil War at 19, was a member of the American Communist party and a passionate supporter of human rights. "I read Hons and Rebels at 14 and it changed my life. I named my daughter Jessica after her, and gave her that book as a christening gift."

As the train speeds along I think about Harry Potter - a good-natured, popular hero living in the tragic shadow of his parents' death, brought up in the security of an ancient boarding school. Book seven will see him come of age - and face his destiny - certain parallels occur to me? I ask Rowling her views on the Monarchy? She laughs: "Oh my God! I can't believe you said that! My friend always warns me about inteviews - Whatever happens, he says, for goodness sake DON'T let them get you going on the royal family!"

But alas the train has pulled up in Edinburgh.

Harry Potter and the Philosopher's Stone, Bloomsbury £4.99. Harry Potter and the Philosophers' Stone (adult edition), Bloomsbury £6.99. Harry Potter and the Chamber of Secrets Bloomsbury, £4.99.