Tell us a bit about the character that you're playing in this adaptation of your book.

I play the dinner lady Mrs Trafe, who is quite kindly even though all her food is disgusting - she makes things like chilled pilchard soup. But she's an ally of Joe, the billionaire boy of the title, even though she ends up asking him for money.

That's a bit of a theme for young Joe, isn't it?

I think that children often believe - and adults do too - that if they were rich it would be the end to their problems. But I imagine that in a lot of cases it would create a whole new load of problems: being in a normal school and being a billionaire would be very, very tough; anyone with any money issues would come to you and it would change your relationships with everybody. That's what the book is really about, that wishful fantasy of being rich and finding out that actually, it's not the solution to your problems.

You have said that the script is bigger and better than the book, is that because you can have a bit of a play with it?

The great thing about getting a chance to adapt your own book for the screen is that you get another go at writing it, and you can iron out some of the problems you felt might have been there, but you didn't quite have time or couldn't find solutions to. I've loved working on the adaptations and with other great writers to bring them to life.

In Billionaire Boy there is a lot in the adaptation that isn't in the book - Joe has a celebrity butler (played by Warwick Davis) and there is another lovely sub-plot involving one of his teachers. Both are complete inventions for the TV version.

As they are on at Christmas you may be attracting a much wider audience than you are with the books. Does that bring some enjoyment in the process?

Because it's going to be on at Christmas time we're hoping for a family audience. The books are aimed at eight to 12 year-olds but I know some grown-ups do like them - I read children's books myself, they're my guilty pleasure. The fun is getting a brilliant cast together who can all bring it to life. Of course you hope that the millions of people who love Catherine Tate or John Thompson or Warwick Davis will watch it. And yes, it's got to work on some different levels because it's got to work for very young children, younger than eight, and it's got to work for the grown-ups too. I think the best things for kids normally appeal on lots of different levels; you can go and see Toy Story with no children present and have a really great time, because it's a smart piece of entertainment.

It's a star-studded cast from a British comedy perspective. Is it easy for you to attract this people into your productions?

The comedy TV world is actually quite small, so you tend to know most people or have met them. We're lucky because this is the fourth adaptation of one of my children's books and the others went well - Mr Stink was even nominated for a Bafta. So hopefully it's something good, that people wanted to get involved in.

You filmed in Sheffield and Rotherham and used some local school children in the production. Surely it's going to be the Christmas of a lifetime for the ones who see themselves on screen?

Well, when I was 11 or 12 I wanted to do acting, but I had no idea how to get into it - I didn't know anyone who did it and it seemed totally remote to me. Some of the kids we've chosen go to the school, and for some it was their first ever job. They're going to be on the big Christmas promo and the cover of the DVD and everything, it's exciting for them and I'm sure they'll go on to be actors as they're all really talented.

The thing with kids is, you've got to work with [unknowns] because by the time they've done something great and you've heard of them, they're often too old to play the part you want, because there's a huge difference between say, being 11 and being 13.

You've talked a bit about your character, but why did you want to play that particular role? Was there a dinner lady who inspired you at school?

Actually, the co-writer Kevil Cecil suggested I play Mrs Trafe the dinner lady - I wasn't sure because I've never dressed as a woman before and I didn't know if I'd enjoy it, but it fitted well and it's a good comic part for me.

I did have quite a scary dinner lady at my school called, I think, Mrs Pierce. She had a big ladle and she'd go, "I'll wrap your knuckles faster than you can say Jack Spratt", if you made comments about her food not being very nice - she was quite terrifying. That was the kind of era where you were forced to eat food you didn't like, like rhubarb crumble. If you didn't like rhubarb you'd be crying trying to eat it, sitting on your own after it had gone cold, tears welling up and dripping into your bowl. So, I imagine dinner ladies now are lovely and this is maybe a bit of a throwback dinner lady.

I'll probably get a lot of comments from dinner ladies, because that always happens when you write a book. "I'm a dinner lady and I'm appalled how you've portrayed us." That's what happened when I wrote Demon Dentist, every time I go to the dentist they complain. You can't win if it's comedy.

If you made a billion pounds what would you spend it on?

I've met some billionaires and they have pretty fun things, like private jets and boats and houses all over the world... but I'm not going to say any of those things. If you gave a lot of it to charity, I think that would give you the best feeling. But it would be fun to have some of those toys, wouldn't it.

Do you have any more inventions in your mind, apart from Bum Fresh? If there is one thing you could invent in real life what would it be?

I think Bum Fresh is actually a really good invention - toilet paper that is dry on one side and wet on another. I think someone should come up with that. I don't know if I've come up with any other good inventions. My mind doesn't really work like that. That toilet paper is about the only one.

What are you getting for Christmas?

I don't know. I normally just ask for inexpensive things like a book. There's a new biography by Ted Hughes that I'd quite like. Probably quite boring presents really.

What will you be watching at Christmas?

I'll be watching myself mainly. I don't quite know what's on yet, there's almost too much on, too many great things. You have to argue with members of your family because they circle things in the Radio Times and that clashes with that film and you can't watch it.

What I really like is watching a James Bond film in the morning, because they always put one on, and a Carry On film late at night. Just as you get into bed on Christmas night. Carry On Camping is on at midnight, I'll watch that. I like that the schedule goes all over the place and there's things at odd times. And I watch the Queen's Speech as well. I quite look forward to that.