In his new book The Music Room William Fiennes did not formally identify the family home as Broughton Castle, near Banbury. “I wanted people reading the book to imagine the castle for themselves without necessarily knowing where it was.

“However, I know that people living in the region will easily work out where our home is. And some newspapers did accompany their reviews with photographs of the castle. If I had any control over that, I would not have included the pictures,” he said.

Rather, he wants his book to be seen as portraying a family — any family — that has to learn to cope with someone with epilepsy. His brother Richard, older than William by 11 years, had a severe form of epilepsy. “I hope the book will be a contribution towards a conversation on a much misunderstood condition,” said William.

Judging by the reception he has received from reviewers and literary festival-goers this year, he has succeeded. “I have had a very good response from readers so far,” he said.”It is about a family coping with a difficulty and it is also the story about myself and pain.

“I wanted to release the book from being purely a documentary. It is about a family that could be anybody's family, although a family living in an unusual setting,” he explained.

As well as telling the story of Richard, it includes extracts from medical reports on research into epilepsy over the centuries.

“The severity of epilepsy seems to depend on the degree of damage to the brain,” he said.

Richard had scarring to the frontal lobes behind the forehead. He died in 2001, aged 41. William is the youngest of the five children of Lord and Lady Saye and Sele. His eldest siblings are twins Martin and Sussanah and another brother, Thomas, died in a road accident while riding a horse, before William was born. The actor brothers Ralph and Joseph Fiennes are distant relatives and by coincidence Joseph starred in Shakespeare in Love, which was partly filmed at Broughton Castle.

The Music Room is Williams second book. His first, The Snow Geese, was awarded the Hawthornden Prize in 2003 and was short-listed for the Samuel Johnson Prize in 2002. Just as The Music Room has sprung from his own life, so had The Snow Geese.

“When I was recovering from a bout of Crohn’s disease, I read Paul Gallico’s book The Snow Goose and decided to trace the migratory journey of snow geese from the Gulf Coast of Texas to the Arctic. I think I put a lot of love in the book and it is also about lifelong experiences.”

After The Snow Geese, he tried several topics for his next book, but never seemed to connect with the words on the page and eventually decided he needed to write about something about which he had strong feelings. It was then he started thinking about the family’s experiences with Richard.

“I began writing about Richard and growing up in the castle about five years after he had died. By then the shock and rawness of his death had receded and I felt like a salvage diver going down to the seabed to bring up the treasures of memories before the sea washed over them,” he said.

William felt flattered when I suggested The Music Room would do as much to explain epilepsy as Mark Haddon’s novel The Curious Incident of the Dog in the Night-Time had done for people’s understanding of autism.

“I am a great admirer of Mark’s book,” he said.

As well as his writing career, he is also founder of the charity First Story, which encourages teenagers to write creatively.

William was inspired by a teacher friend, and by a similar project in America founded by the author David Eggars.

First Story began two years ago with William working with children in a secondary school in Hounslow, west London. “The charity has been very successful and last year we were in 16 schools. This year we will be going into three schools in Oxfordshire,” he said.

The charity picks secondary schools where there is a high proportion of pupils receiving free meals and where the examination results could be better.

“I found children love to write. They are not writing classical English literature, but they are encouraged to be original,” explained William.

The charity is funded by charitable trusts and private donations, with finance needed to fund writers in residence and to publish anthologies of the children’s work.

* The Music Room by William Fiennes is published by Picador at £14.99.