It’s a story which has everything – murder, mystery, passion and even magic. Yann and Sido rescue each other against huge odds, ranging from the evil Count Kalliovski to the start of the Terror, when the ideals of Robespierre, Danton and Marat were lost in a welter of blood.
“I love periods of history that are like dysfunctional families,” she says.
Gardner’s discovery of her own powers as a best-selling novelist came about in almost as strange and heart-rending a way as Yann’s, in that she has overcome two major handicaps.

The first is that she was born with severe dyslexia, undiagnosed until she was 11. Her inability to read meant that she was sent from school to school including a boarding school for maladjusted children. Her father was a QC and an MP, and her mother – England’s second woman judge – firmly refused to believe that their daughter was stupid, but (this being the 1960s) nowhere would take her, because she couldn’t read. Badly bullied and thought to be mentally handicapped she lived inside her head, “where I had a huge, complicated story going on.”

Dyslexia, which she now sees as a great gift, “is like catching a train that’s meant to take you from A-B, and instead of following the journey you’re looking out of the window and seeing fields full of flowers, and perhaps getting off to pick them. No teacher can put you in a category, so it becomes crippling.”

It was at boarding school that she discovered her power as a story-teller, making her dormitory shriek at her ghost stories, – and it’s hard not to feel some sympathy for those terrified schoolgirls when you come to the hair-raising Sisters Macabre in The Red Necklace. Like Dickens’s A Tale of Two Cities rewritten by Angela Carter, this is a tale steeped in wickedness of a ferociously enjoyable kind.

Gardner left school at 16 and blossomed in Art School, winning a first class degree, prizes and Arts Council awards to do theatrical design. The arrival of three children by her successful designer husband meant switching to work as a picture book author and illustrator, producing such enchanting works as her Book of Princesses, The Glass Heart and Fairy Shopping. It was only when her marriage unravelled and her husband left for New York that she found herself writing for her life.

Characters in Gardiner’s stories have an intimate knowledge of suffering acute reversals of fortune, and it’s hard not to think that their power and sympathy are drawn from experience. Left with acute financial troubles, she got a contract from Orion for her first novel, The Strongest Girl in the World. Armed with this, went to her bank to beg for an overdraft.

“I’m not going to sink, and I’m going to survive,” she said; and because she didn’t break down in tears, they backed her with a two-year overdraft, which she repaid by becoming a best-selling author.

Her Magical Children series is much-loved by younger readers, but it was I, Coriander which announced the arrival of a unique voice and vision  and won the Nestle 2005 Gold Award. It is a gorgeous, riveting read set in the time of Cromwell which girls of 10+ fall in love with, but The Red Necklace with its bloody intrigues will appeal even more to boys. Perhaps the biggest surprise is the sheer power of her prose. Take Yann’s progress through the first day of the Terror:

“It was as if they were welded together like one determined, monstrous body made up of flesh, sinew, teeth and hair. With one purpose, with one mind, with murder beating in their hearts, they moved inexorably forward. They had no past, no future; they were caught in the great unthinking moment, their hearts and minds driven wild by the frenzied ringing of the tocsin, the firing of the cannons. It was as if Paris itself had a voice and howled its terror for all to hear.”

Despite her dyslexia, she has steeped herself in contemporary accounts of the French Revolution, and her theatrical background gives an understanding of the drama of public executions.

“The guillotine made it too quick to satisfy the public,” she explains. “Its efficiency as a killing machine made it simple to execute many priests and nobles in one day, giving the people the spectacle they wanted.”

Living in Stoke Newington, in a small but exquisite house whose interior is like a fusion of Versailles and Coriander’s faerie refuge (complete with stuffed alligator and crystal chandeliers), the delightful Gardner is now as ebullient as she was once crushed. Like her heroine, Sido, she has mixed feelings about the Revolution, pointing out it was the birth of everything modern in our world. “Equality between men, between women and men, it was like a virus that spread, the greatest thing ever,” she says. “We’re still battling the issues of how to live today. The storming of the Bastille was a vision of liberty, the end of the feudal system, the finest moment. Then it all gets completely polluted.”

Ends

The Red Necklace by Sally Gardner is published this week by Orion, £9.99