Jamila Gavin’s novel The Blood Stone describes a journey that can’t be done any more, overland from Damascus to Afghanistan. As it is, her young hero Filippo, Catholic son of a 17th-century Venetian master jeweller, and his Muslim protector almost perish several times while crossing the desert to avoid their pursuers in Baghdad and Basra.

Their first destination is Agra, India, where they hope the Great Moghul Shah Jahan will buy Filippo’s family’s treasure, a rare diamond called the Ocean of the Moon, and give them the money they need for the jeweller’s ransom. Shah Jahan has not yet built, or even thought of, the monument for his empress known as the Taj Mahal; it would be impossible to find the rooms in the Grand Vizier’s palace where Filippo and Khan are kept in luxurious house arrest as the emperor’s enemies bid for the jewel. You can’t point out the stretch of water off Crete where a storm blew them perilously off course, or the spot in the desert where they drank their dead camel’s blood to survive.

But if you trace their path back to Rio san Lorenzo in the Castello district of Venice, you can see the house from which Filippo set off.

Probably.

Ms Gavin was writing a history of the Taj Mahal for a project that later fell through when she found the name of Filippo’s father, Geronimo Veroneo, in footnotes to accounts of the building of the monument. He was a guest of Shah Jahan and was held hostage by Afghans. “One account says he died in Nepal and another that he was buried in Agra.” This “tiny shred of material” is real in The Blood Stone; everything else is invented, especially the implication that Veroneo’s Ocean of the Moon diamond, set in a fabulous pendant, inspired the design for the Taj Mahal.

No proof has been found that Veroneo made any impact on Shah Jahan’s architectural masterpiece, Ms Gavin admits. “But on the other hand, nothing has been found to refute it. He was around sufficiently for his name to be linked. The Taj Mahal is very like a jewel in the way that it changes with the light through the day.”

Into her story she added a fake Ocean of the Moon (Filippo has the real one sewn inside his skull, where it serves as a third eye to warn him of danger and show him the future), a mercenary and master of disguise whose loyalties are as elusive as his accent, a stack of secret agendas and an extended Veroneo family complete with an evil son-in-law and a spy in the kitchen. Like the fake diamond, it’s not true, but you’d never know the difference.

Once in Venice, a city built for subterfuge, spying, hide-and-seek and wheeler-dealing, the tale seems even more plausible. Beneath the present-day tourist-friendly gloss, and accompanied by Gavin, it’s obvious how it would have all worked.

From the house on the Fondamento san Lorenzo, near Ponte Leon, you can see the church of San Giorgio dei Greci, which Filippo’s friend Andreas, son of a Torcello fisherman, attends with his family. You can trace the route the Veroneos take, reluctantly, to meet their despised in-laws, the Pagliarinis, for mass at Santa Maria Formosa, and where Filippo and his brother Beppe walk to deliver drawings at Campo San Pantalon, where Beppe is killed in a scuffle with the Pagliarini sons. The fish market where Andreas sometimes sleeps under his father’s stall is intact, so is the Angelo bridge, where the Pagliarinis’ unhappy servant finds poison for their guard dogs, left by the mysterious benefactor who later whisks Filippo’s hostage sister into the night.

Which is just as well, because the author didn’t know any of this when the story formed in her head. “I had seen enough of Venice so that I could envisage the network, and I knew you could get anywhere from the Grand Canal. Later I checked things like whether you could have heard sandpipers in Venice then. But I had no particular buildings in mind.”

Her friend Liz Foster-Hall, a guide who has been visiting Venice for 10 years, helped the geography slide into place once the book had been written.

She had friends who lived in “just the house” for the Veroneos: a light-filled workshop on the top floor, family rooms below (although the neighbourhood has gone upmarket, with a grand hotel and designer shop across the canal).

Ms Foster-Hall found a suitably grandiose high-walled palazzo for the Pagliarinis, and tracked Filippo’s routes around the city (having done her Blood Stone tour, I have just one complaint: that Filippo didn’t seem to stop for a cappuccino). “Every time you imagine something and find out it could be true, it’s wonderful,” Ms Gavin says.

The result is that The Blood Stone has a strong sense of place equal to that in Coram Boy, the Whitbread prize-winning novel set in Handel’s London, and the Surya trilogy set in India during Partition. Jamila Gavin’s memory has always served her well. The village in the Punjab near Amritsar where she spent parts of her early childhood (she was born in India in 1941) was recreated in fiction before she had the chance to visit again: the palace her family lived in while it was being turned into a teacher training college, the bungalow they moved to, the road, the lake where she remembered a child drowning.

“That was one of my earliest and most distinct childhood memories at about three or four; a photographic rather than an emotional memory. I didn’t go back for 50 years; when I did, I was amazed at how much I’d got right.”

The other half of her childhood was spent on visits to her English mother’s family in west London, slotted into a convenient primary school each time.

On a return visit four years ago to St Saviour’s primary in the west London borough of Ealing, “the school where I learned my times-tables”, she was surprised at her strength of feeling. “It was a perfectly fine school with no horrible teachers and I had no memories of a bad incident there, yet standing in the playground I felt the anxiety and despair of the first day when I was seven. I couldn’t believe how upset I was. It was a complicated time. Most of us were on the poverty line and at school we were being given food parcels from the United States. Going back to India on the ship in the late 1940s I remember gorging on bananas, I was allowed a whole bunch to myself. They hadn’t been seen in England for years.”

Yet bomb-blasted London, she believes, is the place that first made her a writer: “Before I went back as an adult, I had remembered St Saviour’s strongly, for my fascination with the holes in the ground and the broken buildings overgrown with weeds and brambles. The church behind the little school was blasted out and the arches were still there. To me it seemed a magical fairytale setting; it stirred my imagination.”

Imagination still stirred, she’s now writing a book on Alexander the Great, so there’s another journey to be done. At least in her head.

The Blood Stone by Jamila Gavin is published by Egmont Children’s Books; a paperback edition is due next month