Only a brave man would consider writing a sequel to Robert Louis Stevenson's classic adventure Treasure Island. Sir Andrew Motion, former Poet Laureate and UEA lecturer, tells STEVEN RUSSELL why he did it.

Sir Andrew Motion is best known for his 10 years as Poet Laureate and biographies of Philip Larkin and John Keats.

Now he's gone dramatically off-piste by dreaming up a sequel to Treasure Island – a rare venture into fiction that he says absolutely energised him. Silver: Return to Treasure Island takes us to the London of 1802. Near the Thames lies the Hispaniola inn. It's run by the grown-up Jim Hawkins – who as a lad narrates Stevenson's Treasure Island.

With him in 1802 is son Jim junior, who in the taproom listens to tales of adventure on the high seas: stories featuring revenge and buried treasure. And a man with a wooden leg . . .

Late one night, a mysterious girl called Natty brings a request from her father – the aforementioned pirate Long John Silver, last seen in South America, escaping with a stash of treasure. Older and frailer, he wants his daughter and Jim junior to sail to Treasure Island and find Captain Flint's hidden 'beautiful bar silver'.

The crucial piece of the jigsaw is a map locked away in the Hispaniola inn. Natty persuades young Jim to steal it and join her in pursuing the silver. All initially goes well, but it later appears that Treasure Island is not uninhabited, as thought.

For Andrew Motion, who was head of the famed creative writing course at UEA before becoming Poet Laureate, the notion of writing a novel for children came out of the blue, though he relished the chance.

After the death of his agent of 30-odd years, Andrew talked to a prospective replacement who said 'You spend much of your working life in schools; why don't you write something for children?'

'So I did,' he recalls. 'I'd never tried to write action before, but I thought I'd see if I could do it and if I had any appetite for it. Anyway: absolutely loved doing it.

And, by the way, I don't think this is a children's book, though I think children will enjoy it,' he adds. 'Just because you write other kinds of stuff doesn't mean you can turn on a tap and write something else.'

Didn't he have qualms about attempting a sequel to a classic story? 'For a very long time I've been a fan of Stevenson. Who isn't? Especially boys, I think. He himself wrote a sequel to Kidnapped [Catriona] – which nobody much reads these days but it's not actually a bad book – and I've often wondered whether he didn't leave some doors open so that he might go back himself, one day; but then didn't live long enough to do it.

'Specifically, what I mean is that he leaves silver on the island. The loot taken away is doublets and jewellery and suchlike, and gold, but all the silver is left there. He told us that, in fact, on the very first page of Treasure Island.

'He of course allows Long John Silver to escape at the end. He also much less noticeably gives him a wife who is left behind in Bristol, running a pub. So it's not beyond the bounds of reason he may have worked his way back to England and hooked up with her. Third, and the thing I find most imaginatively exciting, he leaves three maroons [abandoned men] on the island – really wicked pirates he doesn't want to bring back to England. So what happened to them?'

Treasure Island itself is 'something of a mash-up', he points out, borrowing from the ideas of Daniel Defoe and Edgar Allan Poe, among others. 'I feel that gives me licence to mess around with it. That was the thing I felt steadied me when I was having nervous attacks about writing.'

Seeking to avoid the pitfalls involved in writing a sequel, he strove not to make Silver too close to the original.

'A lot of sequels that don't work don't work because they enter into a competition they're bound to lose. I thought pushing it on a generation, making my narrator a different character from his father, I would be able to write something which would kick off from the original book but be its own thing.'

So he never felt the shadow of Stevenson on his shoulder? 'I felt beckoned on by him; encouraged by him – excited by some of the playfulness I could have with his book. I felt it was fun!'

? Silver is published by Jonathan Cape, priced �12.99.