Children’s fantasy author Robert Muchamore left school with a D in economics and now sells a million books a year, but he’s quietly modest about his achievements.

All the Crouch Ender will say is that it was “nice” to find success with his Cherub series, about an espionage unit of teenage orphans who bust unsuspecting terrorists and international criminals.

“I was never very academic and didn’t want to go to university. I was the kid who spent a lot of time sitting in corridors for making silly noises in class and generally underperforming,” says the 42-year-old, who grew up in Junction Road and attended Acland Burghley.

After school, he spent 12 years as an investigator tracking down the heirs of intestate wills.

It was when nephew Jared hit his teens that Muchamore got the idea of writing for children.

“He was very bright, but I was frustrated because, being quite a bookish person, I saw he didn’t engage with books at all and, when I looked at what was on offer, I didn’t blame him.

“I was always a kid who read but found it frustrating after nine or 10 to find books I wanted to read.

“Post-Harry Potter, publishers gained so much confidence that they could make money out of children’s books but back then there wasn’t much for that awkward age when you are not really an adult but not a kid any more.”

He instinctively knew that “fantasy: punch-ups, girls and soap opera infighting is what kids enjoy reading” and invented intelligence officer James Adams as a better looking, braver alter ego. “He was the person I wanted to be when I was 12. All James’ likes and dislikes are mine. He likes Arsenal and hates mayo and ketchup, but he is the cool version of me, good looking, blonde, tough, brave and gets girlfriends. It’s pure wish fulfilment.”

For two years he didn’t make enough to quit his job – his first advance was less than £5,000 – and he would work 7 to 3 then go home and write in “an adrenalin rush because I wanted to make a success of it”.

On the bus trips home, he picked up the vernacular of teenagers.

“When the bus stopped outside Holloway school it was flooded with kids running up and down the stairs fighting and swearing. I was quite irritated at the time but in terms of writing it gave me a real sense of their dialogue – so that’s what I wrote, the 390 bus minus the swearing.”

Bit of luck

Luckily Anthony Horowitz’s child spy Alex Rider was starting to take off just as he sent his story out to literary agents, who were looking for similar properties.

“Cherub was quite different and purely coincidental. I knew nothing about Alex Rider but, no matter how talented, writers need a bit of luck and that was mine.”

Once he sold the film rights, it gave him the push to quit work.

“I enjoyed my job but it was nice to have a new career. I feel lucky I had a job I quite liked, then did something else and became successful.”

He’s now written 13 books plus a prequel series, the ’40s-set Henderson’s Boys which imagines the wartime work of the secret agent who set up Cherub.

“I didn’t realise how much extra work it was to write historical fiction – if your character walks into a house, you have to spend half a day researching 1940s curtains, floors, whether it would have had running water and an inside toilet.”

His latest book, Rock War, (Hodder £12.99) took Muchamore further into unknown territory – the murky world of rock journalism and talent shows.

Having never seen The X Factor – or played more than a recorder – he watched an entire season of The Voice and scoured NME to master the lingo.

“I was useless at music lessons and when I wanted to do a book about kids in rock bands, I realised I had to find out how you write about music, describe sound or a performance in a compelling way. I read music reviews and bought classic books by rock journalists to pick up the terminology.”

Over the four books, one character will become hugely famous and another slips happily back into their old life, allowing Muchamore to play about with the genre conventions

“The talent-show thing seemed a bit cheesy but gave me a chance to bring these bands together competing and jostling for position, to play with perceptions of how glamorous it is and how much of it is faked – although you can’t be too sneering or superior because kids love these shows.”

Staying in touch with his inner 12-year-old, remembering what it felt like “to be acutely self-conscious and not know who you are or where you fit in” is all part of the job, says Muchamore, who points out that writing for children involves a different skill set to adult fiction.

“A lot of adult authors have tried to write for kids and completely failed. You have to get into their mindset, remember you are writing for people whose emotions and attention spans are different. Kids have this incredibly intense feeling that everything in the world should be right and just. I get furious emails about perceived unfairness. Ultimately they regard having a happy ending and the good guys winning as right.”

In 2011, his books were banned from Highgate Junior School after parents complained of racy content.

“I don’t set myself up to be controversial and I am not pushing the boundaries of indecency. But if you are writing for 11 to 14-year-olds, you are competing with violent movies and Xboxes, and cosy stories aren’t going to lure them away.

“People hold children’s books up to an artificially high standard.

“With my publisher, I’ve set unofficial guidelines – mild sex and swear words, some sexual references but no explicit content. My rule of thumb is the EastEnders test – would this happen in an episode of EastEnders?”

Muchamore lived in the same area for years with his mum, a cleaning lady, and dad, who sold insurance.

“I only moved out four years ago to Crouch End. I went a bit posh,” he says with a wry smile.

He doesn’t have children but says that helps his writing because, “being a parent, you get this overwhelming sense of love and protection while I don’t see children in the same way – I can be cool uncle Rob.”

Libraries

He recently got cross about fellow authors Francesca Simon and Michael Rosen talking up doom and gloom about the publishing industry and library closures.

“When I go to Portugal and France they are in awe of British children’s literature. It’s a global force. Britain really has a cultural dominance in children’s books; we should wave the flag and be proud, not be moaning about how terrible it is.

“It’s not that I hate libraries, but if I was the local councillor and had to choose between meals on wheels or mental health care and the library staying open, I would probably do the same as them.”