A literary wizard for adults

At one point Strange helps the Duke of Wellington beat Napoleon by moving the city of Brussels to America, while Mr Norrell creates a phantom fleet of English ships to blockade French ports.

Norrell resurrects the young fiancée of a Cabinet minister (not entirely successfully), while Strange rescues the mad King George from the clutches of two cruel asylum keepers.

In context, this doesn't strike one as oddly as it should. "One way of grounding the magic," says Clarke, "is to put in lots of stuff about street lamps, carriages and how difficult it is to get good servants."

The Austenstyle drawing rooms and Dickensian street furniture root the book in time and place, while the silver bowls of water and centuries-old books of spells (many of them explained in the detailed footnotes) belong to Clarke's fantasy England, where fairies with "thistle-down hair" practise the wildest and most reckless forms of magic.

Now 44, Clarke was always determined to write a novel. Her parents were Methodist ministers and she spent her childhood roaming northern England, moving from one congregation to another.

For her first eight years out of Oxford University, she worked for large publishing houses and wrote short stories, but eventually "got rather fed up with London and never having any money", so jacked it in to teach English as a foreign language in Turin and Bilbao. "But then I felt I ought to come back after two years. My writing and my cultural roots were here, and I needed to be here, too.

"Magic is something I've always loved since early childhood," she says. "Partly it's the escapism, the being in another world."

As she began to construct the bizarre parallel universe of Jonathan Strange & Mr Norrell, she signed up for an Arvon writing course and there acquired not only a new confidence in her work, but also a new boyfriend, the science-fiction novelist Colin Greenland, who was one of the tutors.

Impressed by a short story Clarke submitted, Greenland sent it to Neil Gaimon, author of the Sandman series of fantasy novels, who in turn passed it onto a short-story magazine. Interest in her work, and her putative novel, began to grow.

"Colin says he always realised the most important thing about me was this book," she says. "And that's probably pretty lucky. I think very few men would allow that." For the next 10 years, Clarke worked as a cookbook editor for a publishing company in Cambridge - editing the recipes of Antony Worrall Thompson, among others - and got up early in the morning to write.

"What happened when I was writing Strange was the world kind of changed in my favour," she says, referring to the rise of Harry Potter in the bookshops and Lord of the Rings at the cinema.

She grins at the mention of Harry Potter. She thinks the comparisons are merely a form of shorthand used by publishers and booksellers to promote her novel (but of course isn't complaining about it).

"On the most basic level, yes, if you're an adult and you like reading about magic, then maybe this will interest you. But could this be a Harry Potter for adults in the sense that Harry Potter was a phenomenon?

No, there isn't going to be another Harry Potter in our time. And in any case I don't think books can possess you when you're an adult in the same way they can children.

However much you love a book and can't put it down, adults move onto something else in the end, whereas Harry Potter will become the furniture of the minds of a whole generation of kids. That kind of huge influence isn't going to be mirrored by any adult book."

Clarke hasn't had time yet to have children of her own.

"I don't think I could have done this and had kids too. Well, I could have done, but I'd have been a very cruel and neglectful mother." She has, of course, been able to give up the dayjob - it's reckoned that Jonathan Strange & Mr Norrell will eventually make her a millionairess, though the advance was nowhere near that mark - and last year began another novel.

"I developed an entire world while I was writing," she says, "and I still seem to know a lot more about that world than I put into Strange & Norrell. There was this sense sometimes when I was writing that I was just skimming the surface and that I could write lots and lots more about it. So the next book is set in the same world, but focusing on different characters."

At another mammoth 800 pages? "Oh no, much shorter," she replies - and then grins. "Which is exactly what I said the last time ..."

Jonathan Strange & Mr Norrell is published on 30 September by Bloomsbury.