With more than 20 best-selling Horrid Henry books to her name, Francesca Simon was made an offer most writers can only dream of by publishers Profile and Faber And Faber.

“They said ‘Anything you want to write we will publish’,” she recalls. “I thought it would be a chance to do something different.”

Split between a modern world where Christianity never uprooted pagan beliefs and the mystical world of the Norse gods, The Sleeping Army is certainly different.

The story follows schoolgirl Freya who, by blowing a note on a ceremonial Viking horn, wakes the sleeping army contained within the Lewis Chessmen on display in the British Museum.

Accompanied by two of Thor’s young bickering bondsmen and a violent Beserker, she is transported to Asgard – home of the Norse gods – only to find the immortals are under an ageing spell and losing their powers as a result.

They entrust Freya with the quest to release the enchantment within nine days – or join the ivory chessmen herself.

“I have always been interested in medieval stories,” says Simon, who studied medieval history and literature at Yale and Oxford.

“The sleeping army story applies to countries all over the world – England has King Arthur’s army sleeping under Tintagel, China has the Terracotta Warriors, Hungary also has an army that is asleep until the kingdom needs them.

“I had the Lewis Chessmen as the army and the girl who calls them, but the big problem was the reason why they needed to wake up.”

It was her close friend Emily Woof – one of the two people the book is dedicated to – who came up with the solution.

“The way I write is I ask myself a lot of questions, and the answers are the book,” says Simon.

The key question turned out to be “What if Christianity had never happened and modern Britain was pagan?”. Simon created a modern world where the Norse gods, as worshipped by the Anglo Saxons, are the principal religion.

“It was so much fun working out the details,” says Simon. “The Lewis Chessmen became Woden’s sleeping army. The ecclesiastical centre is York, not Canterbury, names from Norse legend, such as Freya, replace Biblical names.”

Simon’s alternative history features a different take on Henry VIII’s Reformation, a new calendar and references to “the gods” in times of trouble.

In a similar way to the Church Of England today, worship of the old gods is in decline – although Freya’s mother is something of an evangelical preacher railing against Richard Dawkins’ polemic The Gods Delusion.

It’s a world Simon has explored in more detail in The Sleeping Army’s sequel – The Lost Gods, which was released in hardback in September. The sequel sees Norse gods Woden, Thor and Freyja descend to the Earth with the aim of encouraging more people to follow them, so they can attain enough power to fight off the Frost gods who have been released by the melting ice caps.

Their methods include appearances on The X Factor, the catwalk and the professional football pitch.

The Sleeping Army may be set in a mythical realm but Simon has ensured it is still based in reality through its main protagonist Freya – a typical modern schoolgirl with interests in make-up and her mobile phone, a hatred of PE and a reluctance to be a hero. “Often the most heroic people are not the ones you would pick to be heroes,” says Simon.

“People don’t know what’s inside them. I wanted Freya to find something inside herself, to make her rise up to the challenge.”

Simon also hopes the books might encourage her readers to discover the Norse myths.

“I don’t understand why the Greek myths are more popular,” she says. “The Norse myths belong more to these islands.”

Accompanying Simon’s words are illustrations by Brighton’s own Adam Stower.

“I like to give illustrators their head,” says Simon. “I thought the pictures should be of objects, not people, but I didn’t give him potential subjects.”

At the end of each chapter is one of the Lewis Chessmen, while each chapter title-page features a preview of what is to come – from dragons and giants to wolves and ravens.

Simon is already working on the third in the Lost Gods series – The Monstrous Child. With The Sleeping Army set in Asgard and The Lost Gods set in modern-day London, she admits all bets are off for the location of this next story.

The Sleeping Army was her first book aimed at an older age group – from eight upwards after requests from older Horrid Henry fans.

“I generally never think about the age I’m writing for,” admits Simon. “I usually write the book and see who is it for.”

But Horrid Henry fans need not despair.

A new book – Horrid Henry’s Crazy Ketchup – is set for release at the end of June.

  • Young City Reads 2014, across Brighton and Hove, Thursday, March 6, to Thursday, May 22