'Everyone was madly attracted to my dad Roald Dahl - he had enormous sexual charisma'

Neal was often working in America, starring in films such as Breakfast at Tiffany’s, so Dahl would look after the children, ferrying them to school before heading to his hut at the bottom of the garden to write.

Lucy, now 50, reminisces joyfully about that time – particularly night-time visits to the woods, where her father would make mugs of hot chocolate for the children and wrap them in blankets for a trip to see “Mr Badger and Mr Mole”. To stop them being bored (“He absolutely hated children being bored. He used to say boredom was death”), Lucy remembers how he once bought a Morris Minor for them drive around a track he had created in their five-acre orchard. "We spent our days zooming around and never crashed!"

Mealtimes were equally as eventful. The children grew up on a diet of Minpin eggs (quails eggs from Harrods), giant eggs from the BFG (duck eggs) and red cabbage that they were told had been delivered by footmen from Buckingham Palace sent by the Queen. “We believed all of this stuff,” says Lucy. “I never stopped believing in it.”

Dahl, who died 26 years ago from a blood disease, is survived by his second wife Felicity – whom he married after divorcing Neal in 1983 – and his four children. On Sept 13, 100 years since his birth, global celebrations are marking Roald Dahl Day.

He lived and worked at home – it was where he was happiest,” Lucy tells me when we meet in London’s Soho House. “When I was young, I was close to him because he was always there. He would write in his hut at the bottom of the garden, and disappear into the world of his characters.”

Dahl’s storytelling has bewitched millions of children across the world, but it all started with his own brood. When they were young, he would send them to sleep with stories of witches who hated children and big friendly giants who collected dreams – all before these characters made their way on to the page and became the classics we know today.

  “The only time he was a bit grumpy was if ever he lost at the horse racing,” says Lucy. “He’d also be annoyed [if he was interrupted] in his hut. It was his dream world – he used to call it a nest. If we ever interrupted him, it was that same reaction: 'What do you want? You’ve just woken me from a dream’.”

Dahl grew up in South Wales to Norwegian parents. Educated at boarding school – his worst teachers there are combined and immortalised in Matilda’s Miss Trunchbull – he never went to university, working instead for Shell Oil in Tanzania. When the Second World War broke out, his lust for adventure led him to join the Royal Air Force. 

He began writing after his active life ended when a crash left him with hip and spinal injuries. But it was more than a decade before he became established as a children’s author, with James and the Giant Peach, at the age of 45.

Today, Lucy thinks he would be “secretly very chuffed” with the fuss around his centenary, though he disliked “notoriety”. That reluctance to avoid the limelight could be why he never told anyone that he was a spy during the war and slept with countless high-society women while gathering intelligence in the US.

These glimpses into his other life only emerged in a biography by Donald Sturrock, written 20 years after Dahl’s death. “When it came out, I felt like saluting him,” Lucy says. “I’m amazed he kept his mouth shut.” His womanising, though, was less of a surprise to his daughter. 

“People were madly attracted to him. Even the girls at school would put their make-up on when he came to pick me up because everyone fancied my dad. He had this enormous sexual charisma. Women melted when they saw him.”

Lucy has a family of her own: Phoebe, 27, a fashion designer, and Chloe, 25, a restaurateur, from her first marriage to Michael Faircloth – a water-ski instructor whom she married, aged 22, in Florida. She is also aunt to the model Sophie Dahl.

Though she raised her daughters in Los Angeles, where she worked as a screenwriter, she tried to give them a taste of her own childhood in Buckinghamshire.

“I did the same things, but the American version. I’d wake them up with a big midnight feast and surprised them one day by taking them to Disneyland instead of school.” Both of Lucy’s daughters are gay, as is her sister Ophelia, but she is reluctant to discuss this. “I do draw a line with my children. I get very maternal instincts.”

However, she will say how thrilled she was when Chloe announced her engagement to fellow restaurateur Nikki Booth, her girlfriend of three years. “We were out riding recently with Nikki, and Chloe said, 'Mum, would you mind if we eloped?’ I said I’d be delighted, because I know how much weddings cost.”

She also tells me how much she can see of her father in both daughters. “They say it skips a generation, and I see immense talent in both of them. Dad would have been very proud.”

Lucy divorced her daughters’ father in 1991, and last month finalised her second divorce to John LaViolette, a lawyer. “I’m a bit embarrassed about that,” she admits. “Dad used to say it’s forgivable to be married two times because everyone’s allowed one mistake. But a third marriage? There’s a problem somewhere. The things you learn from your parents when you’re young stay in your heart.”

It’s why she is “determined” to never marry again. “Unless he’s rich, doesn’t want a prenup and is old.”

She never took either husband’s surname, something she’s now pleased about: “If I were to advise any young woman, I’d say keep your name purely to keep your identity. It is important women keep their own identity.” Her full name is Lucy Neal Dahl, and the reference to both her parents reflects a strong part of her identity.

“I think about my father every day – and my mother. I talk to them both all the time. I feel they’re around me at different times. If I need courage, I call upon my mother. If I need creativity, I call upon my father. If I’m scared, I call them both. I hope one day I hang around my children, too.”