WHEN they made a film of Vikas Swarup's bestseller, they gave it an extreme makeover. But can I get the author to say anything critical about Danny Boyle's hit adaptation of his debut novel, about a penniless orphan who wins India's Who Wants to Be a Millionaire? Not a chance. Swarup, you see, is a diplomat. And not just any diplomat his sumptuous business card, embossed with three golden lions, tells me he is minister and deputy high commissioner of India, based in Pretoria.

They changed the title from Q&A to Slumdog Millionaire. (“That made a lot of sense,” says Swarup.) They changed the ending. (“Danny thought the hero should be arrested on suspicion of cheating on the penultimate question, not after he wins as I had it. That was a successful idea.”) They made friends into brothers, axed Bollywood stars and Mumbai hoodlums and left thrilling sub-plots on the cutting-room floor.

Crucially, they changed the lead character's name from Ram Mohammad Thomas to Jamal Malik, thereby losing Swarup's notion that his hero would be an Indian everyman, one who sounded as though he was Hindu, Muslim and Christian. Instead, they made Jamal a Muslim whose mother is killed by a Hindu mob. (“It's more dramatically focused as a result, perhaps more politically correct.”)

“I was forewarned of the changes by Simon Beaufoy, the screen-writer,” Swarup says. And he's still happy. “The film is beautiful. The plot is riveting. The child actors are breathtaking.”

Swarup has one niggle. He worries how that scene of Hindu mobs murdering Muslims will play when the film opens in India next week. “People in India are sensitive about how they're portrayed, so there will be criticisms. But a Bollywood director recently told me Slumdog Millionaire's failing was that it wasn't extreme enough to be truly Indian. India has a genius for recycling its contradictions.” Swarup rewards my sceptical frown with an endearing smile.

But why would Swarup complain? From the window table of our restaurant in London's Victoria, bus after bus rolls by advertising Slumdog Millionaire. He points them out. His debut novel, already translated into 37 languages and garnering awards around the world, is back in the bestseller lists. And Swarup is basking in the glow of the four Golden Globes that the film won this week. Not to mention the 11 Bafta nominations. Paulina, our waitress, notices his novel on the table and tells me she loved the film. “It was about real struggles against adversity,” she says. “It really spoke to me.”

Born in London

Fair enough, Paulina, but what you don't know is that the Slumdog Millionaire from Mumbai's meanest streets was born in London's rather more genteel Golders Green. He came to life on Swarup's laptop while the diplomat was finishing his British tour of duty at the Indian high commission in 2003.

“I had two months left in London before I went home,” recalls Swarup, 48. “My family [wife Aparna, sons Aditya and Varun] had already returned to Delhi, partly because our children were not really rooted as Indians. We had been in Turkey, Washington, Addis Ababa and London and it was time to go home. My eldest son supported the England cricket team. His hero was Andrew Flintoff. Terrible!

“After they had gone, I thought 'Now is the time to write the novel.' But I'm not one of those writers who wants to spend four pages describing a sunrise. There are so many of them in India. I'm a sucker for thrillers and I wanted to write one. I'm much more influenced by Alastair MacLean and James Hadley Chase. I'm no Arundhati Roy.”

A catalyst was Major Charles Ingram, convicted for cheating his way to winning the British version of Who Wants to Be a Millionaire? “If a British army major can be accused of cheating, then an ignorant tiffin boy from the world's biggest slum can definitely be accused of cheating.”

Swarup's conceit was that an uneducated hero becomes a contestant on Who Wants to be a Millionaire? and, through the sort of miraculous fortune that would make an atheist believe in a benevolent personal deity, is asked a series of questions that he can answer. Ram's success makes everyone suspicious. How can a slumdog know who Shakespeare was? Q&A's retort is that Ram's adventures in orphanages and brothels, with gangsters and Bollywood celebrities, have taught him the answers to each question posed. The novel's seductive opening sentences are “I have been arrested. For winning a quiz show.”

Swarup is the second Indian novelist to have hit the headlines recently with a slum-dwelling chai-wallah hero who gets rich quick. The other was Aravind Adiga, whose novel White Tiger won the Booker prize last October. Like Adiga, Swarup is from a middle-class Indian family. His parents were lawyers in Allahabad. His grandfather's library, which little Vikas ploughed through, had a first edition of Mein Kampf next to Isaiah Berlin's Two Concepts of Liberty. Swarup is many things, but no slumdog.

'An urban myth'

“This isn't social critique,” he objects. “It's a novel written by someone who uses what he finds to tell a story. I don't have firsthand experience of betting on cricket or rape or murder. I don't know if it's true that there are beggar masters who blind children to make them more effective when they beg on the streets. It may be an urban myth, but it's useful to my story.”

Swarup knew that he had to complete his novel before leaving London. “I'd been made India's director of relations with Pakistan. It was going to be 9am to 9pm every day. So I had to finish the book before I got the plane home.”

He wrote quickly - one productive weekend yielded 20,000 words. “It was only with the 11th agent I sent chapters to that I got anywhere. I e-mailed Peter Buckman the first four-and-a-half chapters on Wednesday. On Thursday he wrote back. The following week we met. He told me he wanted to sell the book. The only problem was there was no book.”

On September 11, 2003, however, he handed the first draft to Buckman. Soon afterwards, Buckman negotiated a six-figure two-book deal for his client with Transworld. “I am the luckiest novelist in the world. I was a first-time novelist who wasn't awash in rejection slips, whose manuscript didn't disappear in slush piles. I have had a wonderful time.”

Like Adiga's White Tiger, however, Swarup's novel is unlikely to win plaudits from the Indian tourist board. Its depiction of Swarup's homeland is hardly diplomatic. “You might think that, but I have had no complaints, not from the Mumbai police [whom he depicts as child torturers] or from anyone in the government. My country respects artistic freedom.”

Before Q&A, Swarup's last published story was written half a lifetime ago. It was called The Autobiography of a Donkey. No one yet has optioned the film rights. “Maybe I only had one great idea that everybody can enjoy the story of an underdog who wins. I'm not so sure I'll ever be so lucky to come across another story.”

His second novel, Six Suspects, was published last year. But this complex, Indian-set whodunnit has a major problem. “The problem is my nine-year-old son. He claims to have read Six Suspects. He told me he wanted an MP3 player for finishing it. I said no. Now he threatens that if I don't he'll name the murderer from my book on Facebook.”

Our time is up. After lunch, Swarup must fly back to South Africa. Is it difficult to be a writer and a diplomat? “I can't write in the crevices of a working day. So it's hard.”

Does he dream about giving up the day job? “No! There's no better time to be an Indian diplomat. India is flavour of the season. While the rest of the world is going to hell, India and China are doing well. I revel in my job.”—Dawn/Guardian News & Media