Cultural exchange … Blake meets the Australian performer Tim Minchin, whose adaptation of Dahl's Matilda as a musical was inspired by the illustrator's work.

QUENTIN BLAKE is about to turn 80 - his birthday is next month - but there has been no let-up in his work-rate: he still goes at it like a maniac, scarcely ever taking a break.

But if the artist, who famously illustrated the books of children's author Roald Dahl, still works at the same hectic pace, his emphasis has changed somewhat. The main thing is that he's been doing a lot of murals for hospitals and mental health centres, which also appear in a new collection of his recent work, Beyond the Page. There are now Quentin Blakes on the walls of a Chelsea mental health centre in London, an eating disorders unit in Victoria and a children's hospital in Paris.

As well as allowing him to work on a much bigger scale, these have sent his imagination roaming into hitherto unexplored territory.

''It's been a wonderful new challenge. Each job is different, of course, but I suppose what I've tried to do in every case is bring some colour into what might otherwise be pretty monochromatic lives.''

Everything Blake does takes shape in one room of his flat; a large, rather gloomy space with shelves crammed with books of his illustrations. In many ways he seems so unworldly you don't expect him to have an entrepreneurial bone in his body. Yet a glance at the floor suggests otherwise. In one open cardboard box, I spot row upon row of Quentin Blake oven gloves. You can also buy Quentin Blake greetings cards, fabrics and wallpapers - all of which must have made him an extremely rich unworldly man.

On the other side of the room, by the window, is his drawing board. Blake takes me over to have a look, padding across the carpet in his white shoes. There are bottles of ink and jam jars full of quill pens, some made from feathers of vultures, some from turkeys (fans now send feathers from unusual birds for him to make pens out of). Everything is arranged neatly enough, yet there's still an air of chaos. A sense of exuberance waiting to be unleashed.

This exuberance is present in everything Blake does. It's one of the things that makes his work instantly recognisable. With the minimum of pen strokes, he can convey character, mood and, perhaps most strikingly of all, movement. The night before our interview, I read one of his books to my six-year-old son. When I asked him what he thought of the drawings, he said, approvingly, ''Very lively.''

The same could be said of another of Blake's new ventures - a series of female nudes entitled Sporting Girls, which he has done for an exhibition at the Marlborough Gallery in London. These girls are big - very big - and they cavort about with legs kicking and breasts bouncing. For all sorts of reasons, you don't normally associate children's book illustrators with eroticism. Yet it's plain that Blake, in the right mood, can draw a sexy nude with as much panache as he can an Oompa Loompa.

''Ah,'' he says, beaming. ''I'm pleased you liked them. I suppose people might be a bit surprised. They do tend to assume that because you do children's books, everything must be bland and optimistic. I mean, I've got a pretty strong line in optimism myself, but there are other things to life, aren't there?''

But there's something else that makes Blake unique among illustrators, and it might just be the secret to his success. He never seeks to impose his vision of how a character should look. He always allows space for the readers to fill in the gaps with their imagination.

''What I try to do is create a kind of tolerance that the mind of the reader can work within,'' he says. ''When I draw a character, I try to make it defined, but not to close it up completely. It's as if I'm a go-between, between the writer and the reader.''

For as far back as he can remember, Blake has been drawing. The son of a civil servant, he grew up in Sidcup, London, with a pencil in his hand - or so family legend has it. Does this imply that he was a solitary child?

''Mmm, solitary-ish. Not that I didn't have friends. In fact, I still have friends that I had at grammar school. But I didn't do sport, or anything like that. Far too busy drawing.'' He gives an unexpectedly hearty laugh.

He drew at Cambridge, too, where he studied English. By then he had already begun contributing cartoons to Punch. He had started sending in cartoons: a rough version to begin with, then a more polished one if he got a commission.

''The art editor told me one day he thought my roughs were better than my finished drawings. That was quite a turning point for me. I realised I had to let myself go a bit.''

One of the other myths about Blake is that he started drawing for children because he felt a special rapport with them.

''No, no,'' he says, waving a hand dismissively. ''Nothing like that. I know some children's writers write for specific children, or for the children they once were, but I never have. I just thought children might like my sort of visual humour.''

He never had a plan of action; he just told himself he'd carry on until he was 30 and then take stock.

''Then I realised I'd passed 30 and so I just kept going.''

He turned out to be a natural collaborator, mainly, I suspect, because he doesn't have much of an ego. ''I've always thought that the writer is the front end of the horse, as it were. It's useful if you get on personally with whoever's written the words, but I don't think it's essential. Someone like [poet/author] Michael Rosen, for instance - we first worked together more than 30 years ago. I see Michael, but we never go to each other's homes or anything like that.

''However, I remember once overhearing him talking about our collaboration. He said, 'It's very simple. I give the book to Quentin and he just goes off and does it.' I was very encouraged that he felt he could trust me in that way.''

Of all Blake's collaborations, it's the one with Dahl that he's best known for. It was a partnership that got off to a far-from-promising start.

''I think we first met in Tom Maschler's office when Tom was running Jonathan Cape. Maschler had asked Dahl why he'd never done a picture book, so he'd gone off and written The Enormous Crocodile. I did that and The Twits, but then we rather came apart over The BFG.''

Being tight with money, Maschler only commissioned 12 illustrations. Dahl wanted far more and flew into a tremendous rage when he saw the proofs - ''and Roald was someone who knew how to be angry'', Blake says. ''He thought I was being lazy, when in fact I was just doing what I'd been asked to do.''

Dahl fired off a furious letter to Maschler: ''This is cheese-paring to the ultimate degree … It is also an insult to my book … I will not agree to your publishing The BFG unless properly and fully illustrated in the same manner as all the others.''

Maschler backed down and Blake started again. Soon afterwards, a parcel arrived at his door. ''When I opened it I found a large sandal.'' This, Dahl wrote, was what the BFG should be wearing, not the clumsy knee-length boots he had been in.

It proved to be a perfect partnership, in large part because Blake's natural optimism and gentleness balanced Dahl's much less forgiving view of the world. I wondered if he'd ever been shocked by the nastiness of Dahl.

''Not in person because we always got on very well, but in the books … Yes, once or twice. I was a bit shocked by The Witches. That's pretty frightening and it did take me back a bit. Also Roald dealt in revenge, which I don't approve of.

''But the only time I said anything was when we did Revolting Rhymes. I thought it was a bit too rude and I wrote and said, please remember that parents read these to their children. I thought he might object, but it was fine.

''There was also a problem with one of the poems in which a wizard went off with Sleeping Beauty. It seemed to me she was a bit young and Roald hadn't given her enough time to grow up. I pointed this out and he said, 'Oh no, we can't possibly have that - it's paedophilia!'''

As their collaboration went on, the two men developed a routine. After Dahl had written each book, Blake would go down to his home, Gipsy House outside Great Missenden, where they'd discuss how the illustrations would look.

''I think we knew how each other's minds worked. And we respected one another a great deal.

''I remember when I was doing the illustrations for Matilda, Roald told me the way I'd drawn her wasn't how he'd envisaged her, but then he said, 'It works, so let's stick with it.'''

With Dahl, Blake usually had a pretty clear brief to work to, but a lot of the time when he starts drawing he doesn't really know what he's going to do. When he looks at what he's done afterwards, he often wonders where it came from.

And just occasionally when Blake is drawing, he has a very strange sensation. ''I do every so often have this odd feeling that I'm meeting someone else while I'm doing it.''

His inner-self?

''I suppose so,'' he laughs. ''Although I don't know what my inner-self would be like.''

Telegraph, London

Beyond the Page by Quentin Blake (Tate Publishing, $39.95).