Francesca Simon has just finished walking her Tibetan spaniel, Louis, around Tufnell Park where she’s lived for the past 20 years. They’re both spattered with mud. The dog’s boisterous behaviour could certainly give Francesca’s notorious literary creation Horrid Henry a scamper for his money. But she disagrees.

“No!” she laughs. “He’s just a very sociable puppy.”

Henry, the mop-headed mini-tyrant with a loathing for his soppy younger brother, Perfect Peter, has been a publishing phenomenon. Twenty-one million copies have been sold in the 22 years since Henry first began rampaging through the middle-class home he shares with the infuriating Peter and their exasperated parents.

Henry has run amok in 29 languages. In particular, they love him in Spain where he creates mischief as ‘Pablo Diablo’. Francesca explains his appeal: ‘Children as young as four instantly realise that it’s all the parents’ fault.

“They have created the problem by idealising one child over the other. My books are very open about the fact that Henry absolutely hates his brother and would do anything to get rid of him.”

Crucially, she believes her stories give children the confidence to express “uncomfortable emotions in a comic way.”

Visit any public library and look at any of the 96 Horrid Henry books available, you’ll see that the pages are invariably covered in ketchup and chocolate stains – proof of how many times these much-loved stories are borrowed. Francesca says: “It’s fun to spend time with a rule-breaker such as Henry.”

Indeed, such is the power of her stories that she says she knows of at least two families who, while listening to audio tapes of Horrid Henry stories in the car, narrated by the actress Miranda Richardson, they have caused such a distraction that the driver almost had a road accident.

It was the arrival of her son Josh (now 26 and a budding theatre director) that was the catalyst for Francesca’s writing career. Although she had written as a child, she says she was ‘flooded with ideas’ observing north London parenting.

“If it hadn’t have been for Josh, I wouldn’t have known I had a flair for writing children’s books.”

Born in Missouri in the US in the mid-50s, Francesca’s childhood was bohemian and peripatetic.

The eldest of four siblings, by the age of 12 she had been to seven different schools in America, France and London. Her early childhood near the beach in Malibu, California, was, she says, “idyllic” but “squished”.

Her father, Mayo Simon, screenwriter of the film The Man From Atlantis and the “go-to” man for sci-fi projects always seemed busy, tapping away on the typewriter. “Our house was all very compressed. I shared a tiny bedroom with my sister, Ann [now a professor and the scientific advisor on TV’s The X-Files].

“I didn’t get on with my brothers and sisters back then. I just felt: ‘Please will you all go away!’ I wanted to be an only child.” And so Francesca retreated into the world of books and began writing what she describes as her own “fractured fairy tales” which were inspired by the fairy tales of the Scottish writer Andrew Lang.

Modestly, she says: “I did the classic novice-writing thing of never finishing them.”

Arriving in London in her early 20s after a spell abroad, she lodged with the biographer Claire Tomalin in Camden’s Gloucester Crescent. After studying at Yale University, Francesca took up a place at Oxford University to study medieval history and literature, alongside Tomalin’s daughter.

“I thought: ‘Wow! This is London! This is my haven.”

She says she often bumped into Tomalin’s neighbours, Alan Bennett and Jonathan Miller, but meetings with such cultural giants were wasted on her. She jokes: “They might as well have been the butcher and the baker.”

At the time, Francesca was a student of leading Anglo-Saxon scholar Professor Eric Stanley, who himself had been a pupil of JRR Tolkein. Indeed, she has returned to this ancient world after the success of her two mortal gods books for older children, The Sleeping Army and The Lost Gods with a new title for young adults called The Monstrous Child, due to be published in May. It is the bleakly funny saga of Hel, the Norse goddess of the dead.

Francesca describes Hel as “no people-pleaser. I see her as an angry teen who is locked in her bedroom, which she’s painted black. She feels hideous.

She rages: ‘I am ugly. Everyone hates me. I hate everyone’. It’s intense, chilling stuff – but myths and legends do have a lot to say. There’s a reason they have survived; they speak to very deep parts of us.”

A new departure for Francesca is to have written The Monstrous Child in the first person.

She explains: “I was on the subway in New York when I suddenly got this voice in my head, which turned out to be the first sentence of the book. I have never felt like I was channelling a voice before but I could instantly get Hel’s voice.”

At school she was something of a Perfect Peter herself - “you never met a better behaved child,” although she claims she was “horrid” at home.

These days, her only trouble-making involves mischievously writing Horrid Henry scenes that she knows will challenge her friend Miranda Richardson when she narrates them for the audiobook – for example getting her to sing snatches of high opera.

But she says it doesn’t work – “Nothing fazes Miranda!” As Francesca Simon wisely says: “Everyone is a combination of Horrid Henry and Perfect Peter, aren’t they?”