"I always think that the background or wherever you place something has to be a character in itself," she says. "The absence of character in a place is wrong unless you're doing something particular with it. [London] has such a huge character and I've lived here all my life so I know it like the back of my hand."
Reminiscent of Philip Pullman's His Dark Materials trilogy, The Door That Led To Where
examines the impact the past can have on the present - or indeed the devastating effect the present can have on the past, as AJ discovers the key to a mysterious gateway that allows him to travel back to the 19th century.
"If we don't study history, we shouldn't be surprised if we end up repeating the same old mistakes," says Gardner. "History is the greatest map we have to see what we should be avoiding. It's the only thing we have left of our forefathers, which can guide us into becoming more clever and intelligent."
Gardner focused on the French Revolution in her 2007 novel The Red Necklace and its 2008 sequel, The Silver Blade. "What really interests me are periods of history when history becomes dysfunctional, as something extraordinary happens while at the same time a mass of people end up being murdered.
"It becomes a very riveting thing, where a lot of clever, young people came up with these issues, such as women's rights, that are still valid today."
Gardner explores these themes again in The Door That Led To Where as AJ befriends Esme, a young woman he crosses paths with in Victorian London. While Esme's plight is still very relevant today, Gardner was determined that it would also feel very authentic for the period.
"When I did The Red Necklace, I was accused of making the main character Sido too weak, as she didn't take enough ownership of her situation," she says. "But it was the 18th century and Sido was not meant to be like Katniss in The Hunger Games with her bow and arrow.
"She's a marquis' daughter, and it's amazing what she manages to do, but I can't stand it when we police the past."
With AJ and his friends Slim and Leon using the door to the past as an escape route from their problems in the here and now, Gardner suggests not much has changed in the intervening 200 years.
"I don't think there's too much difference between them," Gardner says of the two time periods. "But I thought that they would be given a new lease of life there, which they are never going to be given today. It's something that I'm always asking: 'Where are young men meant to have adventures nowadays?' We've got this society now where we either villainise men or we mummify them, so Syria and places like that end up looking like Boy's Own magazines, especially if you haven't studied anything."
With AJ leaving school with only one GCSE in English but with a deep appreciation of literature, Gardner draws comparisons between him and his favourite author, Charles Dickens, who was similarly self-taught.
"The thing about AJ is that he's got a terrible home life, so how do you work when you're life is so grim?" says Gardner.
"You look at people now who get divorced and they say that they can't work, but kids grow up with this all the time and they're expected to take exams, be very bright and go to Oxford and Cambridge."
A guest at next month's New Zealand Festival, Gardner will soon travel to Wellington, where she will appear at Writers Week alongside a diverse selection of international authors that also includes Jasper Fforde, Miranda July, Andrew O'Hagan, Richard Dawkins and Patrick Gale, as well as local luminaries such as Anthony McCarten, Witi Ihimaera and Paula Morris.
"I've been so lucky, as I've done so many of these kinds of events recently," she says. "In the last two years, I've seen the whole world, or not seen very much of it all as might be the case. But it's a great thing to do, and on the whole, the cultural exchange is really good."
Writers Week, part of the New Zealand Festival, runs from March 8-12.
