Philip Hensher on his weighty new work

Hensher's ripostes range from, "Don't be fat, no-one likes a fat author" to "Make sure your top and trousers match" and "Before going out in public try to develop some personal charm."

Well, when we meet, 49-year-old Hensher is not sporting matching top and trousers, although he has personal charm. He is, however, somewhat hefty. As is his new novel, The Emperor Waltz, which weighs in at a gargantuan 614 pages - a fat book then, but one packed with delights as Hensher's prose dances light-footedly across time. In fact, there are so many surprises in the novel that it's still surprising the likeable Man Booker-shortlisted author himself.

He begins by telling me this when we meet over abstemious glasses of fizzy water in a London club of my choosing, but which it emerges was one of the places Hensher and his husband, Zaved Mahmood, a United Nations human rights lawyer, considered as a venue for their marriage in 2009. "Too seventies, we decided," says Hensher, easing his ample girth into an armchair in the bar overlooking Green Park. So the twitter of birds, drifting through open windows, becomes the soundtrack of the interview.

Which is fitting since Hensher's novel has a striking crimson cover, with an image of a blackbird in full song perched on a twig.

"Ah, the blackbird," says Hensher, who divides his life between homes in south London and Geneva, where Mahmood works. "Do you know, I didn't notice that there was a recurrent blackbird until the publishers came up with this cover. Now I realise there are a lot of them in the novel. I've no idea why, but it must have been something in my mind. So I went through the book again at a very late stage - and I even put a couple more in."

The motif of the blackbird is just one of many symbolic, buried treasures in this novel, from the repeated image across the decades of a red-headed woman sweeping her long hair aside, revealing the curve of her neck, to Strauss's Emperor Waltz, often heard by Hensher's characters, be they gay rights campaigners in 1970s London or Bauhaus artists, such as Paul Klee, in 1920s Weimar as the shadow of Nazism spreads across Germany. This gay - I use the word in its original sense - melody echoes across the 20th-century Europe that Hensher has conjured up so vividly in a trilogy of apparently disparate stories that make up his magnificent novel, with its sweeping narrative and cast of unforgettable, Dickensian characters. Indeed, the novel is as satisfying as a monumental 19th-century work of fiction.

"Yes, that's exactly what these repetitions are - buried treasures. It's nice to hear that's how you see them," says Hensher, when I tell him how much pleasure the reader gets from unexpected treats, such as the repeated imagery and the haunting sounds that have led one reviewer to describe Hensher's ninth novel as "masterful" and "dazzling" and "glorious."

The Emperor Waltz is all of the above. It opens with the Bauhaus artists, teachers and students before moving to late 1970s London where a young man invests a legacy in a gay bookshop shortly before a mysterious illness starts spreading through the gay community. Then we're in a London hospital - next year perhaps - with the author himself miserably confined to bed with an infected toe. We're also transported to AD203, where a beautiful, young Christian woman - St Perpetua - sacrifices her life for her beliefs.

So, three coteries at odds with the mainstream of their culture and communities, trying to change hearts and minds?

"Yes, absolutely, although I never wanted this book to follow an orthodox plot," explains Hensher, who made Granta's best young British novelists list in 2003 and won the Royal Society of Literature's Ondaatje Prize 2013 for his last novel, Scenes From An Early Life. "Partly a life story, partly a novel and partly a history," it tells of his Dacca-born husband's childhood in the 1970s before East Pakistan became Bangladesh, and is about both Mahmood's family and the brutal war of independence.

"I always like those books where you just feel that, like life, things come into it and out again and you never know what's going to happen, so that's what I set out to do with The Emperor Waltz," Hensher reveals. "It's strange, but this book often surprised me as I was writing it, although it's been on my mind for a long time. I think it all began with the gay bookshop. I sent the German bits to a friend, a noted German historian, to check and he asked if I remembered when we were students, in London in the 1980s, walking past Gay's The Word [the London bookshop devoted to gay literature]. Apparently I said, 'Someone ought to write a novel about them'." (Hensher adds a disclaimer that no-one in his book bears any resemblance to anyone who works or who has worked in the real shop, although real people, such as novelists Angus Wilson, Paul Bailey and Adam Mars Jones, have walk-on roles.)

"So it's been in my mind for a long time," he continues. "As for the Bauhaus chapters, I've always known I'd write about them. I bought the Paul Klee Catalogue Raisonne the moment it came out 15 years ago because I thought, 'I'm going to need that.' Klee is the artist I love best in all the world for his willingness to combat oppression and violence with laughter. So for four or five years I've been thinking of writing about these two separate threads that are somehow connected.

"Suddenly, I realised how much the gay bookshop and the Bauhaus had in common; then I thought I needed a third thread of the story and the thing that people - well, my friends - are not going to expect from me is something about Christians. I am not a very spiritual person at all. It just came together, very easily. St Perpetua's autobiography is such a wonderful story! Incredible!

"It's such a beautiful idea of moving towards openness. I've always been moved by that thing Evelyn Waugh says that Christianity is the first religion ever that doesn't have a secret component, that doesn't carry out its miracles behind closed doors in an inner sanctum, that it's made for doors to be flung open and for light to be flung on it. I also loved how maddening the early Christians must have been. They drove the Romans up the wall because they wanted to be thrown to the lions. They were saying, 'Execute me, martyr me.' They wanted their moment in the sun.

"What I'm interested in in this novel is the way that people's minds change and the way in which a belief held by a small, loathed minority can somehow spread in an inexplicable way."

Change is the great theme of much of Hensher's work, from the gritty realism of his Man Booker Prize-shortlisted The Northern Clemency, an epic about the lives of ordinary people from the early 1970s through the Thatcher era, to Afghanistan-set The Mulberry Empire. But then there's his libretto for Thomas Ades's opera, Powder Her Face, about the scandalous life of the Duchess of Argyle, which remains in the repertoire "because the music is divine".

Brought up in south-west London before his family moved to Sheffield, where his father was a bank manager, his mother a university librarian, Hensher gained a "freakishly good" English degree at Oxford, went to Cambridge to study 18th-century satire, and began writing fiction. He was a House of Commons clerk when his 1996 novel Kitchen Venom - partly narrated by Margaret Thatcher, featuring Commons clerks, rent boys and murder - came out. He was dismissed after giving an indiscreet interview to a gay magazine in which he confessed his desire for Gordon Brown. "Alas, poor Gordon!" he exclaims, although his sacking remains one of the most blissful moments of his life.

Despite being an astonishingly prolific novelist, Hensher also writes non-fiction, such as The Missing Ink: The Lost Art Of Handwriting, And Why It Still Matters (2012), reviews for several publications and is editing two massive collections of 20th-century British short stories for Penguin. He has a teaching gig at Bath Spa University, where he's professor of creative writing. "My students often surprise me, far more than I surprise myself. Some come up with fantastic ideas. I'll think, 'Oooh, I'd like to steal that.' I don't, of course, but I've been tempted," he admits, with a rumbling belly laugh.

The Emperor Waltz by Philip Hensher (4th Estate, £18.99)