WHEN David Walliams was a boy, he used to sit captivated at the knees of his two grandmas as they spun him tales of their lives. The youngster was amazed at how dramatic and exciting their experiences of World War II had been.

Decades later, when the adult David was a successful author, he decided to return to those days of his childhood and recreate that special bond between children and their grandmothers in Gangsta Granny.

“When I was a child I would spend lots of time with my grandmas,” he says. “Sometimes I would selfishly think spending time with them could be boring but when I got them on a subject like living in London during World War II when bombs were raining down, they would become very animated and I would be enthralled. I realised everyone has a story to tell.”

The best-selling novel was swiftly adapted into a stage show and is now headed to Swindon.

It follows young Ben, who is forced to spend every Friday night with his gran while his parents go ballroom dancing – and it’s always an ordeal of cabbage soup and Scrabble.

Then one day he finds a tin filled with diamonds and gems which leads to the amazing discovery that his granny was once an international jewel thief. Persuaded by Ben, they decide to take on the biggest heist ever to steal the Crown Jewels and the adventure of their lifetimes is about to begin.

While David’s grannies were no mobsters, he admits that he did take a touch of inspiration from them.

“There was definitely a smell of cabbages in one of my grandmas' houses. The other did break wind like a duck quacking when she walked across the room,” quips the Britain's Got Talent judge.

“I think grandparents love being grandparents because they get to give the children back to the parents. Children love spending time with their grandparents because they love hearing their stories and being allowed to stay up past their bedtime.”

Born in Surrey, David studied drama at Bristol University before teaming up with Matt Lucas on Little Britain. Initially a radio show, it became a television sensation broadcast in more than 100 countries and scooped a host of awards including three BAFTAs.

A decade ago, his career took a different - and somewhat tamer - trajectory when he decided to try his hand at fiction.

“I had an idea for a story,” he recalls. “What if a boy went to school dressed as a girl? I thought it would be a thought-provoking children's book. That became The Boy in the Dress, my first of eight children's novels.”

His novels have all been hugely successful, topping charts and winning a string of accolades. David now has more than six million book sales to his name, with his stories translated into 40 languages.

Writing for children is a real passion, he enthuses.

“The only limitation in a children's book is your imagination. You can take children on magical journeys in books that many adults would be reluctant to go on,” he says.

“Children love to be scared but it can't be too horrifying. Children love to laugh but it can't be too rude. You always have to be the right side of the line.”

David has been hailed as a modern-day Roald Dahl, his own childhood writing hero.

“I think Dahl's books always feel a little bit forbidden. He manages to balance the humour and scary elements in his stories perfectly. My favourite is The Twits which is utterly hilarious and I love that it is a children's book with no child characters.”

Keen to ensure his own novels prove to be just as memorable for children, 44-year-old David believes his popularity is down to the fact his books are laced with humour and never patronise youngsters.

“I deal with quite big topics - cross-dressing, homelessness, grief etc,” he says. “I know children are a lot smarter than most grown-ups think.”

Two years after it was published, the BBC adapted Gangsta Granny with a star-studded cast including Joanna Lumley, Rob Brydon, Robbie Williams, Miranda Hart as Ben’s mum and David Walliams as his dad.

Following the success of the television film, Birmingham Stage Company hopped on Gangsta mania wagon with a barmy action-packed theatre production.

“It's a huge thrill seeing Gangsta Granny have this whole new life on the stage. There is lots of action in Gangsta Granny, especially when they try to steal the Crown Jewels.

“I especially like the characters of Ben's mum and dad. They have an obsession with ballroom dancing which is very funny live on stage. The great thing about seeing Gangsta Granny in the theatre is the audience gets to share the fun together. It was wonderful at The New Alexandra Theatre in Birmingham on opening night laughing and crying along with everyone else. That's what makes theatre so special.

“The moral of the story is, ‘Don't assume old people are boring just because they are old’,” he says. “In fact they are likely to have had a much more interesting life than yours. Talk to old folk, listen to their stories. They are bound to be full of magic and wonder.”

Gangsta Granny runs at the Wyvern Theatre from September 27 to October 1. To book go to swindontheatres.co.uk or call 01793 524 481.