For 30 years, I’ve been nervous about girls... girls as heroes in books, that is. I’m supposed, after all, to be a writer for boys. Alex Rider, the Diamond brothers, Matt Freeman... these have been my soldiers of fortune, inhabiting a world of guns, explosions, sophisticated gadgets and fast cars, all of which might be considered effectively male. How would a girl fit into all this? Would boy readers accept her? And if she cooked or cried, would I be exposed to charges of sexism? Could I even find my way into the psyche of a 14-year-old schoolgirl? After all, in real life, I only managed to produce sons.

And yet here she is: Scarlett Adams, my first fully-fledged heroine, very much at the centre of Necropolis, the fourth volume of The Power of Five series... not just holding her own against the four boys who unite alongside her but in the end rescuing them. And the strange thing is that, despite my fears, I have hugely enjoyed writing her.

Girl characters have a way of surprising you in a way that boys don’t. Writing her, I found she could be far more mercurial than the boys, close to despair at one moment, violent and decisive the next. She could show her emotions in a way that Alex Rider, for example, would never dare — but this had the effect of making her seem stronger, perhaps even more realistic. Scarlett is a fully-fledged human being. She has parents (albeit divorced), favourite teachers, a boyfriend and a good helping of attitude. From the very start, she isn’t the obvious action hero and the way she gradually becomes one was part of the fun of writing.

Nor is she simply a tomboy, or a boy hiding behind a girl’s name. Take this moment in the first chapter, when she has just escaped death: “She nodded, suddenly tearful without knowing why. Maybe it was just the shock, the knowledge of what could have been. She wiped her face with the back of her hand, noticing that her nails were grimy and all her knuckles were grazed. Her dress was torn. She realised she must look a wreck.” I love the fact that she worries about her appearance after her close escape. I hope it’s psychologically true.

Necropolis was not an easy book to write. It takes place in London, Hong Kong and Peru — and there are alien worlds and twisting time frames. I have five children to deal with instead of the usual one. The action is on an epic scale as the Old Ones, creatures of pure evil, position themselves to take over the planet. At 105,000 words, it’s also my longest book yet.

But in the end I was grateful to Scarlett Adams. If it doesn’t sound pretentious, she somehow guided me through it. I felt completely comfortable when she was on the page and I only wonder why it took me 30 years and about as many books before she arrived.