It's quite appropriate we're talking to you on a train, it's very important in the story.
Yeah I love trains. I wouldn't be here if it wasn't for the fact that my father managed, by the skin of his teeth, to get the train from King's Cross - that's where he met my mother. He proposed to my mother on a train, I had the idea for Harry Potter on a train, yes, very appropriate, I love trains.
Do you sometimes get a bit tired of people saying to you where did the idea of Harry come from, you must get asked that constantly?
I do. I get frustrated with myself more than anyone else. You'd think by now I would have an intelligent and amusing answer to that question, but no I haven't found one yet because the truth is I do not know where it came from, I just don't.
He strolled into my head, fully formed, a scrawny little boy and I knew he was a wizard and I knew he didn't know he was a wizard and I kind of worked backwards and forwards from it. I felt this incredible upsurge of excitement at the idea of writing the story.
Now Harry's got so big do you think it's inevitable in the British way that there will be a backlash against him because we build things up and knock them down so next year we'll be saying - Oh it's not actually that good, we don't like him anymore?
That happens, that happens. I mean obviously I've only very recently really had any dealings with the press and television or anything like that and I've been watching that happen to people I admire for years.
You don't even have to be in the biz to have seen that happen. So to an extent I expected it - on the third book I expected that to happen and it didn't really happen then so I was due.
Is the character of Rita the depiction of your relations with the press?
Well I'll tell you the truth but I doubt very much that anyone's going to want to hear this. I tried to put Rita first in Philosopher's Stone. When Harry walked into The Leaky Cauldron for the first time and everyone said - Oh Mr Potter you're back - I wanted to put a journalist in there - she wasn't called Rita then though but she was a woman.
And then I thought, as I was sort of looking at the plot overall, and I thought that's not really where she sits best, she sits best in four when he's supposed to come to terms with his fame.
So I pulled Rita out of book one and planned her entrance for book four and I was really looking forward to Rita coming in book four.
The first time ever, as I sat down to write book four, my pen kind of metaphorically hesitated to go for Rita because I thought everyone will think that she's my response to what's happened to me.
Well people can believe it or not but the fact is that Rita was planned all along. And did I enjoy her a little more for what's happened to me? Probably I did - I probably did yes.
You put a little more venom in didn't you?
Venom - would you say so? No I wouldn't call it venom.
Now the future. Lupin's going to come back in book five isn't he?
You'll see Lupin again in five yeah, yeah - do you like Lupin?
Oh yes, he's my favourite.
Yeah and me. I always looked forward to writing book three because of Professor Lupin, I love him. You see a lot of old characters in book five. I'm not even going to try and tell you what happens in book five, I'm just recovering from the stress of book four.
You've left us on such a cliffhanger. And how are we with the film at the moment?
It's ongoing, still haven't got Harry which is a bit of a worry. But it's going really well, I've seen some things and they look incredible.
It's the most amazing experience to see - because I've been very lucky, I've been given a lot of input into how I imagined things and they're really trying to recreate what I see inside my head and it's the most extraordinary experience to be able to physically see Quidditch or Hagrid's hut - it's lovely to see what's been in your head for ages, it's wonderful.
Does it annoy you sometimes when the press and people just talk about children's books and they only talk about Harry Potter without realising there's a whole wealth of other children's books out there?
Yes it really does. Children's books have existed for quite a long time in press terms in a bit of ghetto when you look at the coverage that adult books get.
And then you hope that that might change and people say to me - Harry Potter, you know, we want to read it as well - it's a crossover book but loads and loads and loads of children's writers deserve to be and in fact are read by adults.
They might not be quite as famous for it than Harry is, but people like Jacqueline Wilson, David Almond, Aidan Chambers who has just won the Carnegie, Henrietta Branford I really admire but she died unfortunately two years ago, there's loads of people out there - Philip Pullman - wonderful writers.
A few short messages from kids before we finish. Harold Ryan who's 10 from Catford says: "What Hogwarts school house was Hagrid in?"
Err you have to guess because you might be finding out at some point.
"How do you feel about the Americans changing the title of your first book to Sorcerer's Stone?" from Rachel Gummer from Market Rasen.
They wanted to call it something different and I said well how about Sorcerer's Stone as a compromise. In retrospect I wish I hadn't changed but to be honest with you I was so grateful that anyone wanted to buy my book at all that I was maybe a bit too compliant about that.