Here be gargoyles: An interview with William Fiennes

Thomas Fiennes was killed in a road accident while still a young boy, before the baby of this brood, William, was born. The eldest, Richard, died aged 41 after a long and difficult battle with epilepsy. William developed Crohn's disease in his teens and his condition seems especially tough on a writer. "You want complete imaginative freedom," he says, "but when I'm in relapse and suffering diarrhoea a dozen times a day, and there's blood too, and my stomach is constantly hurting, the ability to think and to dream is constrained. It's exhausting and demoralising; I hate it."

Partly because of the Crohn's, Fiennes has taken seven years to publish what is only his second book, but The Music Room has been worth all the pain. He's written a beautiful poem of a tribute to his family, his parents, the magical, moated castle that was his home – and above all to big brother Richard, who loved herons, puns and the damned Leeds Utd in equal measure, and was his hero.

We meet in a bookshop cafe in London, where Fiennes, 38, lives now. To supplement the books, he teaches and is the co-founder of First Story, which sends authors into state schools, some so tough that police vans are stationed outside. Regrettably, he is still single. "It's really quite depressing," he says straight away, before a sip of his hot chocolate. "I'd love to be a dad and to have children. You experience something big and emotive in your life and you want to be able to share it."

Facially he resembles his cousins. He's ineffably polite, remarking that it's "incredibly nice" of me to remember his Crohn's from the last time we met, for the 2002 publication of his debut The Snow Geese. And he describes my noticing that a lot of his imagery in The Music Room concerns death – a viola lying in the plush of its case like a funeral home's newly deceased; Richard, who didn't know his own strength, garrotting taps with plug chains and trussing his Christmas presents as if in readiness for sea burial; drowsiness spreading through him like embalmer's fluid – as "genius". The genius, of course, is all his.

Richard died in 2001, but immediately after The Snow Geese, an ornithological odyssey across the Americas but also a contemplation of home, Fiennes tried to write a novel. When he got stuck, he started another and got stuck again. "That was very despairing," he says. "Naively, I thought if your first book did well, you'd reach this magical plateau of being a writer, and that other books would come easily. The career seemed over. I sent off for the application forms for the civil service; I looked into fast-tracking as a GP. But the real problem was that I didn't care about the characters in those attempted novels. I only wanted to write out of strong feeling, and once I realised that, I knew what to do."

The Snow Geese ended with Fiennes walking up the drive to the family home; The Music Room takes us right inside. And what a house it is. Fiennes learned to ride his bike in the Great Hall and swam, skated and fished on the moat. The King's Chamber, Ladies' Garden, Long Gallery and marvellously named Groined Passage are all steeped in history of their own – 700 years' worth. This is Broughton Castle in Oxfordshire, which once played host to James VI and I and Edward VII and has been in the family for centuries, Fiennes sharing it with regal ghosts, up to 1,000 visitors a day – and these disreputables:

"We opened the gift shop in the stables and I sold Ian McKellen a postcard," he writes. "I ran through the arch into the Ladies' Garden and saw Jane Seymour in a white Regency gown bend to sniff a rose; I was five when Morecambe and Wise came to shoot their Christmas special and I'd been in bed with flu all week, but my mother carried me downstairs so I could see the Great Hall garbed in vaudeville finery, Eric walking over to greet me, adjusting his spectacles and barking, 'Hello! Are you married?'"

The castle featured in Shakespeare In Love, starring cousin Joe. Did Fiennes get self-consciously embarrassed by the grand pile when friends came for sleepovers? "Yes, but that was silly, because there wasn't anything remotely snobby about my parents." He was scared of the gargoyles, the eyes in the portraits, the pike in the moat – but never Richard. "I liked being in the vicinity of that pent-up rage, the day poised on the edge of violence," he says of his brother's mood swings.

Once, he and his mother hid in a bathroom while Richard tried to smash down the door. Another time, Richard held a fork to his father's throat and warned him: "Keep your mouth shut or there'll be trouble." For those of us who didn't know Richard, maybe Fiennes' parents are the heroes of the book. When he found his father leaning against a wall, hand pressed against a buttress, a confused Fiennes discovered the man was asking the house for some of its strength.

"If I had to sum up this book in one word it would be 'care,'" he says. His parents' care for Richard, but also care for the house. "It's ours and more than ours – part of the nation's heritage." Father Nat still runs it at the age of 88 and hopes the book may spark an upsurge in visitors. Mum Mariette, who still polishes the armour, has thanked her youngest son for "making sense of something that had none".

Fiennes adds: "My family home was an incredible place to grow up, a wonderland, but there was also difficulty and sorrow and loss. Every day we read obituaries of people of achievement. Rich didn't make his mark on his world but he was still an incredible person who made a terrific impression on everyone."

The author, who worries that he might have a simile impulse or a metaphor twitch until I reassure him otherwise, seems to fret about a lot of things. "This book is sad but I really hope it's on the side of life," he says. It is. v

The Music Room (14.99) is published by Picador