Roald Dahl's writing hut is just as he left it. Tucked away at the end of the garden, it looks like a dilapidated shed, its paint peeling and faded, its tiny windows dusty with disuse. Inside, the mossy green wing-back armchair in which he wrote nearly all his books takes up most of the space. A drawing by Dahl's son Theo is pinned to the wall, the corners brown and furled. A low table is dotted with objects - a brass ornament in the shape of a Spitfire, a metallic ball made from crushed sweet wrappers. There is a mug filled with sharpened pencils and a yellow, lined pad of A4.

The shed is freezing cold. An electric heater, perilously rigged up to the ceiling with string, has not been turned on since Dahl's death 18 years ago from leukaemia. But there is a different sort of chilliness here too. Everything is in place, exactly as it should be, except the person who made it so.

Felicity, Dahl's widow, does not like to come in here. She refuses to have her photograph taken inside and finds it difficult to spend time surrounded by her late husband's possessions.

When I ask her what life is like without him, she answers with a brutal matter-of-factness: 'It's hell.' Her voice wobbles and she starts to cry. 'Anyway...' she says, flapping her hands in front of her face. The tears disappear. A small, embarrassed smile takes their place. 'As he says in Danny [Dahl's 1975 book Danny, the Champion of the World], you have to be a sparky parent. Well, he was a sparky man. To everyone.'

People feel they know Roald Dahl. Most of us have read his books and had our childhoods shaped by his fantastical mind and macabre sense of humour. Dahl's vision was one of boundless possibility and unfettered imagination; a world where witches had no toes, where giant peaches could float like zeppelins and where friendly giants subsisted on a revolting diet of snozzcumbers.

Sixty-five years after the publication of his first story, The Gremlins, Dahl's books continue to sell at a rate of one million a year. An updated version of his childhood autobiography, Boy, has just been published and several of his other books have been made into films, including Matilda, The Witches and James and the Giant Peach. Charlie and the Chocolate Factory prompted two movie adaptations, the most recent directed by Tim Burton and starring Johnny Depp as Willy Wonka. Wes Anderson is currently filming an animated Fantastic Mr Fox.

Children still pitch up unexpectedly at his Buckinghamshire home, Gipsy House in Great Missenden, where Felicity, 69, lives. 'It's just awful because they look over the gate and say, "Roald Dahl lives here doesn't he?" And I say, "Well he did." "Oh, has he moved?" And I have to say, "No, he died" and it shatters them.'

It is not just children who are intrigued by the Dahl legacy. The December issue of British Vogue carries a series of eccentric photographs featuring Tim Burton and his wife Helena Bonham-Carter in locations and costumes inspired by Dahl. In September, a biography by American journalist Jennet Conant, The Irregulars: Roald Dahl and the British Spy Ring, caused a stir by suggesting that Dahl spent much of the Second World War as an RAF attache in the States, sleeping with wealthy beauties in order to pass on titbits of intelligence gleaned from pillow talk. Dahl's meticulously filed reports had never been published and their intimate revelations raised eyebrows on both sides of the Atlantic.

'Yes, my God!' says Felicity, shrieking with pleasure when I mention the book. 'The sexiest seducer in Washington! But of course it was true. He was wildly attractive and handsome, in his RAF uniform, speaking English, a fighter pilot - completely seductive. And he was charming and intelligent. A lot of women fell for him.'

Felicity, known to her friends and family as Liccy (pronounced Lissy), rarely gives interviews. The requests come in a steady stream, but remembering is an emotive business and she is extremely busy running the Roald Dahl Foundation, a grant-giving charity that aims to help children in the areas of literacy, neurology and haematology. The foundation is about to launch a series of free concerts at Kings Place in London, featuring orchestral versions of Dahl's stories.

Today, however, she has agreed to speak in order to publicise the inaugural Roald Dahl Funny Prize, to be awarded on Thursday to this year's most humorous children's book. The prize, organised by the charity Booktrust, is being judged by a panel that includes Michael Rosen, the Children's Laureate, and Dahl's granddaughter, model Sophie Dahl.

'People often ask me, "Did he tell lots of jokes?"' says Felicity. 'No. It is in his writing, in his descriptions of things. It was a hidden, subversive humour, not a comedian telling jokes.

'Children were his friends, that's what kept him going. The fact that they loved his stories and would then go on to read Biggles and everything else - that, to him, was a miracle. He said, "I feel a bit like a pop star."'

She says he would have been horrified by the erosion of children's imaginations by computer games. 'I think [computer] games are absolutely appalling. A child is never left on their own with nothing, so that they have to create their world. The Game Boys and that ghastly stuff have come in and they are absolutely like this...' She does an impression of a goggle-eyed teen staring at a hand-held screen. 'Roald would have had a fit at that.'

We meet in Gipsy House, an oversized cottage with low ceilings and yellow- and rose-painted rooms. There are photographs of Dahl everywhere and framed illustrations by Quentin Blake hang on the walls. When Tim Burton came to research Charlie and the Chocolate Factory he burst into tears on the lawn. 'People have strong reactions to this house,' Felicity says.

Part of the reason for Dahl's enduring popularity, says his widow, is that he never spoke down to children: 'They were equals.' This, she thinks, was because he never lost his own sense of childish wonderment. He was 67 when Felicity married him (there was a 21-year age gap and she was his second wife, after his first marriage to American actress Patricia Neal), but she says he remained filled with imaginative exuberance. He would produce pink milk for breakfast or make jelly with hundreds and thousands suspended in the gelatine.

'He would make the most mundane thing seem fantastic because he would reinterpret it.' On a trip to Zurich to meet Dahl's European literary agent, they caught a funicular railway and noticed that each time the train stopped at a certain platform, the driver would get out, put his hand up into a ceiling beam and pull out a part-smoked cigar. 'He lit it, had two puffs, put it back and got back into the train to drive down again,' says Felicity. 'All day he did this - up and down. When we got back to the city, Roald bought the most expensive Monte Cristo cigar. We went back up the funicular. At the platform, he took the old stubby cigar out and put the new one up in the beam. Then we went back to the hotel. He didn't wait to see the driver's reaction. That's the sort of guy he was. He was always looking to help people and just make their day a little more interesting, because most people's days were very dull.'

But he could be cantankerous. His granddaughter Sophie once described him as 'a very difficult man - very strong, very dominant... sort of roaring round the house with these very loud opinions'. Does Felicity recognise this portrayal? 'Um...' she lapses into silence for several seconds. 'I don't remember him roaring round, I must say. But, yes, if one of the children was doing something bad, he would roar at them, but with good reason. Also, there were moments he was so ill, and what people forgot was that he was in constant pain from his back injury from the war, and when you're in constant pain you can get ratty. I would describe it as ratty rather than roaring.

'He used to get grumpy when he was finishing a book and I remember saying, "But you should be so pleased you're reaching the end!" And he used to say, "You don't understand - it's the fear of never writing another one."'

Roald Dahl was born in Llandaff, South Wales, to Norwegian parents. His childhood was punctuated by tragedy. When he was four, his older sister died of appendicitis. His father Harald never got over his daughter's death and died a month later of pneumonia. Roald was sent to boarding school at Repton, an experience he remembered in Boy as 'days of horrors'. After school, he joined Shell and was posted to East Africa.

At the outbreak of the Second World War he joined the RAF. His first mission took him into the heart of enemy territory and he was forced to crash land in the Libyan desert, an episode that left him with a fractured skull and the chronic back injury that plagued him for the rest of his life. He was eventually sent home as an invalid but transferred to Washington as an air attache in 1942.

He married Patricia Neal in 1953 and had five children - Tessa, Theo, Ophelia, Lucy and Olivia, who died of measles encephalitis aged seven. Her death was 'catastrophic' says Felicity. 'It was like his father - sort of repeat history.'

There were other challenges too. Theo was left brain damaged when, at four months, his pram was hit by a New York cab. Then, in 1965, Neal suffered three burst cerebral aneurysms while pregnant with Lucy. She was left paralysed and unable to speak. Dahl nursed her back to health. Throughout it all, he carried on writing, producing a formidable body of work comprising 16 children's books, two volumes of autobiography and several short story collections, including Tales of the Unexpected

By the 1970s, Neal was able to walk and talk again, but the marriage was in trouble. While Neal was filming a Maxim coffee commercial, the Dahls were introduced to set designer Felicity Crosland, a divorcee with three young daughters. Soon afterwards, Felicity and Roald began an 11-year affair. When Neal found out, she was devastated, particularly as her own children had known about their father's relationship and tacitly condoned it. Ophelia Dahl, who was 14 when her parents divorced in 1983, said in later years that 'all of us realised that he had found the love of his life with Liccy and there's always a sense of relief when that happens'.

But the immediate fall-out was, says Felicity, 'dreadful because we never thought we could get married, we thought we'd keep it secret. No divorce in the world is happy and I think a husband falling for a younger woman must be the worst of all'.

Did she feel guilty? 'Yes and no. It was a particularly difficult situation because Pat had a stroke and was not well. I don't know how he managed to bring up these children, run a house, do the school runs and write this major volume of work. He was so worn out, so needing to be looked after which of course Pat couldn't do. So there was terrible pain about that.

'It's a very difficult thing. It's tough on everybody.'

Although the tensions of the past have dissipated, it proved especially tough on Felicity: in 1990, her 27-year-old daughter Lorina died of a brain tumour. A few months later, she lost her husband too.

'When Lorina got ill, he was, he was...' She trails off. 'Shock does terrible things to you, physically and chemically. Like Roald's mother, I lost a daughter and a husband within months of each other.'

Now, however, the extended family rubs along happily. The children and grandchildren split their time between Felicity in Great Missenden and Neal in Martha's Vineyard, Massachusetts. 'We're absolutely all very close, including Pat. '

Looking back on the seven happily married years she spent with Dahl, it was, Felicity feels, worth the emotional cost: 'I don't think anybody who met him wasn't taken aback by his whole presence. He was a spectacularly handsome figure; friendly, you know, very welcoming.'

Even at the end, Dahl was still trying to make people's days a little less dull. Felicity recalls that as he lay dying at the John Radcliffe Hospital in Oxford, he noticed that the nurses were being forced to wear their own clothes as an experiment to see whether patients responded better to them. 'He said, "What a ridiculous idea. How can you possibly afford to use your own clothes on the wages you get?"'

Felicity was sent to Marks & Spencer to buy cardigans, jumpers and trousers. 'I came out with bulging bags. He laid them all out and when the nurses came in he said, "Now choose whatever colour you want." That was a man who was dying, he was so ill and yet he still wanted to spread a funny idea to make them laugh and yet to give them a present, give them a treat. Treats were his big thing.'

When we walk outside to Dahl's writing hut, Felicity takes me down a garden path. She stops as the path dips downwards and points to a shiny chunk of bright green jade cut into the stone. She tells me a child from Australia sent him the jade after hearing Dahl on the radio. 'One evening, we were doing our usual tour round the garden and he said, "Do you notice anything different?" I looked up and he said, "Why do you always look up?" So I looked down and there he'd made this little hole in the path and put the piece of jade in with its green side up.' She sighs. 'You know, those are the things in life that matter.'

It is a tale that encapsulates much of what makes Dahl's legacy so lasting: his ability to see things from a child's perspective and to transform the ordinary into something unexpectedly enchanting. It was about remembering to look down when all the other adults were looking up.

The winner of the Roald Dahl Funny Prize will be announced on Thursday. For more information, go to: booktrust.org.uk or roalddahlfoundation.org

Inspired by Dahl: Fan's recollections

Quentin Blake

Has illustrated all of Dahl's books

'We first worked together on The Enormous Crocodile. Our collaboration was like a double act in the music hall. Sometimes you're the straight man and sometimes you're the funny man. Occasionally there were things which were very good to draw and I could actually expand the situation. But there were other bits where the writing was so funny that you just wanted to hold back.'

Michael Rosen

Current Children's Laureate

'In every one of the books, he's on the side of the child. He's thinking all the time about children's desires but he also creates characters that allow children to experience their conflicted feelings about adults. So in The Witches [the 1990 film version of which starred Anjelica Huston, above] the grandmother is the character the boy can bestow all his love on while the witches are the ones he can be angry about. I once met Roald Dahl with my oldest son, and he beckoned Joe over and said: 'What's that growing on your father's face. It's a great disgusting growth and it's probably got yesterday's breakfast in it,' and he went off on a great Twitsian monologue.'

Marina Warner

Writer and academic

'I still remember some of the plot twists in his adult fiction for their amazing ingenuity. The BFG is my favourite of his children's books. It takes a lot of the horrendous scenes about child-eating and breathes humour into them.'

Alan Yentob

BBC creative director

'Whether you are a child or a parent, there are very few writers who make you laugh out loud as consistently as Roald Dahl. There was much of the child in him and in the rather Proustian relish he had for experiences such as eating chocolate.'

Dara O'Briain

Comedian and TV presenter

'I got Charlie and The Chocolate Factory [Gene Wilder played Willy Wonka in the film version, above] for my ninth or 10th birthday. I loved details like the prince who made his whole palace out of chocolate, and the Oompa Loompa song was four pages long! And Dahl never let the moral get in the way of the macabre.'