Emily Diamand's Reavers was the unanimous winner of the first Times/Chicken House Children's Fiction Competition, but she seemed almost too good to be true. Was her surname really Diamand? Was this genuinely her first novel? Was she truly a young mother with an 18-month-old baby, just like J.K. Rowling when Barry Cunningham spotted her?

Yes, to all the above. The surname comes from Diamand's Romanian grandfather, who came to Britain after the First World War. The 37-year-old daughter of a scientist and a teacher grew up in Oxfordshire and has worked most of her life for Friends of the Earth, which may go some way to explaining the brilliantly imagined background to her novel. Set about 200 years in the future when climate change has caused sea levels to rise, Reavers is about what happens when a fisher-girl takes on the corruption of politicians and the bloodthirsty pirates called reavers.

“When I was a child, the big thing was nuclear war,” she said. “Now it's the same for children about climate change. The difference is that I always felt that with nuclear war there was never any hope of survivors. I wanted to write something which showed part of the scenario, but in which people had carried on and could have adventures.”

What adventures they are, too. The heroine Lily realises that the reavers have killed Lily's adored grandmother while she was out fishing. They've also kidnapped the Prime Minister's daughter and Lily discovers a conspiracy involving a jewel that everyone is after. Her best friend is going to be conscripted, and in an attempt to save him, Lily and her cat sail to London (now Lunden) to try to find the missing seven-year-old, Alexandra.

The jewel she steals turns out to have a number of surprises in store, not least that it contains a hilariously neurotic, vulnerable, holographic head believed to hold the secret to military victory. The violent, brutal reavers will soon be under attack from all sides, and only Lily's new friend, Zeph, and the sulking head may be able to save her and Alexandra from a hideous death.

Had this book been sent to me by a publisher, I would have fallen on it with joy. There have been other children's novels set in a flooded Britain, but what makes Diamand's novel special is that it is funny, clever and a towering adventure. Diamand is a genuine 24-carat writer, and Reavers is a gift of a book.

Extract

After the piratical reavers' raid, Lily Melkun, the headstrong fisher-girl, sets out to save the Prime Minster's daughter and prevent all-out war, with the help of a strange jewel-like artefact.

When I get home, I hurtle about getting clothes, oilskins, a knife, extra rope, my rope splicing kit and what food I can find. Which turns out to be a bag of oats and some hard sea biscuits. Well I'll just have to catch the rest.

“I'm going on Mrs Denton's mission,” I say to the empty dark house. Hoping Granny's there somehow, hoping she can hear me. “So I'll have to take the money from the jar.”

I put my hand under Granny's bed and pull out a small jangling jar. Granny's savings jar, where she was hoarding every extra penny for the winter storms, when it's too rough to go fishing. The coins rattle out of the jar and I put them into Granny's purse, which hangs from a loop of leather. I put it round my neck, next to Granny's locket. It ent heavy, there ent many coins in it, but it should last me. After all, things can't cost much in London, can they?

I pat my shirt, where the bulge of the purse shows through. I reckon the purse should be safe from muggers, cos all that really shows of it is a bit of leather at my neck. But what about the jewel? All it'd take is one peek and any thief would be after me. After a bit of thinking, I take out my fishing belt. It's got plenty of pockets for stashing spare line and hooks and all the other stuff you don't want to go searching for when you're out.

I wrap the jewel in a dirty cloth, then I squeeze it into the largest pocket of my belt, where it just about fits. It looks bumpy, but I reckon it'll be safe. After all, who'd ever think there's a big jewel inside a fisher belt?

I've got my bag on my back, and I'm heading for the door, when I catch a glimpse of my reflection in the dark window; round brown face, dark brown eyes, bundled up in stained oilskins, long hair tied back in a ponytail. Girl's hair. But Mrs Denton said she was looking for a captain or a young lad to do her mission. I take the letter out of my bag, and carefully prise it open, trying not to tear the envelope too badly. I read down through her scrabbly writing, and all her fancy phrases. Halfway down are the words that matter.

“I commend this man to you. Please give him any aid you can.”

This man! I ent a man! How will I explain that to Mrs Denton's London trader? What if he guesses I took the letter and the jewel? He'd never help me then.

And that's why I take out Granny's kitchen scissors, use the window as a mirror, and start to cut my hair. When I've finished, I look into the window and there's a boy looking back at me. I lift my hand to my short hair, and he does the same. I open my mouth at what I've done, and he opens his right back. I'm a boy now. A boy who Mrs Denton could have asked to go on a mission.

Reavers will be published by Chicken House in October

Times Recommends

• Superfreakonomics by Steven D Levitt and Stephen J Dubner
• The Times picks the 50 best paperbacks of 2009
• Vincent van Gogh’s letters show the soul of the artist