I found him in the garage on a Sunday afternoon.” The opening line of Skellig is David Almond’s favourite sentence, except for “Catherine Colin Mary Margaret Me”, the names of his family as they appear in his autobiographical collection of short stories, Counting Stars.

The tale of how the small and perfectly formed idea that became Skellig caught Almond unawares in the late 1990s while he was posting stories to his agent is part of as-yet-unpublished writers’ folklore. He was teaching children with moderate learning difficulties at Southlands special school in north Tyneside. He’d been there part-time since 1984, teaching Monday to Wednesday and writing Thursday to Sunday. By the time the success of Skellig in 1998 meant he could write full-time, he had spent 20 years balancing teaching (mostly adult education and special needs) with his quest to get published.

“After the exhaustion of the first few years, it worked pretty well,” he says. He found pupils with special needs especially rewarding to teach.

“The basic difficulties some children have with language is in many ways similar to the struggle a writer has. I learned a lot about myself as a writer from teaching those kids.”

The opening of the story of the mysterious Skellig - human, angel and bird all at once - and his young friends Michael and Mina came “like a present” at a point when he’d had some success publishing short stories but the big break seemed elusive. “I felt someone was tapping me on the shoulder and saying, ‘There you go, son, you’ve been working hard, this is for you’.

“Even while I was writing it, and realising it was a story for young people and that I had a whole new audience, I could feel space opening in front of me. I started to feel I knew where I was going. At the same time I was getting some of the stories from Counting Stars on to Radio 4 and into magazines. There was a sense that something was shifting.”

Skellig won three major literary awards (the Carnegie Medal, the Whitbread Children’s Book of the Year and a Michael L Prinz Honor from US librarians). It’s been translated into 26 languages and sold more than 500,000 copies worldwide. Work on a film, co-produced by the makers of Holes, is likely to start next year.

So far the only hints of what Skellig looks like have been occasional illustrations in foreign editions (“the Spanish one has a quite handsome Skellig, like a matador”) but tonight we’ll find out, when previews open for Trevor Nunn’s production at the Young Vic.

It’s Almond’s own adaptation, yet the tale has taken him by surprise throughout rehearsals. At the first run-through he was “extremely moved to see the story unfold - I managed to forget that I knew what was coming”.

Actor David Threlfall first read the part of Skellig three years ago when Nunn, then at the Royal National Theatre, tried it out in workshops. Almond promises that Threlfall’s performance will be a revelation. “He has an amazing ability to appear both ancient and very young, and he can put his body and voice through extraordinary transformations.”

The play text has developed the relationship between Skellig and Michael (played by newcomer Kevin Wathen) and enriched minor characters such as Michael’s schoolfriend Coot (“I loved seeing him come to life on stage, with the humour accentuated”) and even an old man he meets on the bus, who gets half a page in the novel.

The book is intended for readers of nine-plus but the story’s power draws in all ages. Almond remembers a visit to a school on a Newcastle council estate shortly after publication. “I saw an infants’ class sitting cross-legged on the floor staring up at me. I thought, ‘What am I going to do? I’ve got the wrong group; they’re too young. I read them a few pages.

When the bell rang for playtime this little lad of about five said, ‘We’re going to do Skellig in the yard, Daniel’s going to be Michael and I’m Skellig’.”

Besides an exciting story, the novel Skellig subtly explores aspects of education and creativity. Michael’s worksheet-heavy school day is contrasted with home-schooled Mina’s immersion in blackbird-hatching and William Blake; Michael’s football-mad friends with Mina’s relative isolation. When Michael’s family is in crisis (his baby sister is near death) his writing helps sustain him.

For adult readers, there is also the consoling message that a diet of Chinese takeaways, brown ale and cod liver oil can do no harm. That’s the regime that puts Skellig back on his feet after a period of collapse in the rickety garage at Michael’s parents’ home (based on Almond’s former neighbourhood in Heaton; he’s since moved to rural Northumberland).

Almond’s first book predicts key themes in his four later novels, the Counting Stars collection, the play Wild Boy, Wild Girl, and a new play for younger children My Dad’s a Birdman, which opens in the Young Vic Studio alongside Skellig. It picks up similar themes in the story of Lizzy and her dad, whose experiments with flying win her over. Almond returns again and again to the power of family bonds and friendship to overcome obstacles, particularly loyalty between boys and girls. “Growing up with sisters like mine, I was never in danger of creating weak female characters,” he says.

The shifting boundaries between dreams and reality are also important. It’s all seasoned with the distinct flavour of the north-east and (especially in the second novel, Kit’s Wilderness) affection for its industrial past and Almond’s childhood in Felling, a former mining town a short ride out of Newcastle.

Elizabeth Hammill, director of the Centre for the Children’s Book in Newcastle, and one of Almond’s greatest fans, told an audience during his recent homecoming to launch this year’s novel, The Fire Eaters: “He takes us and our north into the wide world for others to discover and shows powerfully how our world here and the world at large are one.”

That morning he had told 300 children in central Newcastle about the thrill of seeing his first book on the shelves at Felling library with his name on the spine. “Inside me there was this 10-year-old lad going, ‘Yes!’”

That “small square room filled with books” is where he taught himself to be a reader and writer - “It didn’t happen at school, I was thought to be a bit of a shirker there.”

But he adds: “Schools are much better now. The relationships I see between children and teachers are much more rewarding than when I was young. All teachers need is time, and people to get off their backs. Teachers work too hard, kids work too hard.”

There were more children in the audience for the evening event, and you could tell they had read and loved his books when he was asked if he believed in angels (“I believe in imaginary angels; throughout the history of humanity, people have written about creatures who are like us but have wings”) and what kind of creature he would most like to be (“a skylark: as you walked up the hill in Felling, they would jump out of the grass in front of you”).

The teacher in him surfaced, and he returned the child’s question.“What would you most like to be?”

“A dragon,” came the reply.

Skellig and My Dad’s a Birdman are at the Young Vic until January 31. Tel: 020 7923 6363 or see www.youngvic.org. David Almond’s novels are published by Hodder Children’s Books