MURDER, treachery and rebellion are never far from the mind of Ian Mortimer. The author of the best-selling Time Traveller's Guide to Medieval England considers the themes as he takes his daily walk on his beloved Dartmoor.

His obsessions are less the product of a paranoid mind than of a passionate historian who illuminates the dark and foreign corners of the past.

He specialises in portraits of kings, the Henrys, Edwards and Richards, but is also adept at the dung-smelling peasants, lascivious monks, scheming queens and on-the-make princes who make up the chessboard supporting cast.

The murder and treachery comes in when he writes about usurpers and rebels like Sir Roger Mortimer (no relation) who was the subject of his first book, The Greatest Traitor.

Ian, 43, is something of a rebel in the field himself, having upset a few sacred critical cows with his conclusions.

But he is enjoying the last laugh as the Time Traveller's Guide has sold more than 100,000 copies and an Elizabethan version is in the pipeline, together with a new series of fictional works, complete with their own hero and thrilling plots.

In real life he is something of a Renaissance man.

A father of three, married to Sophie, Ian is a musician who has just recorded his first album, a published poet, an honorary research fellow at the University of Exeter and a passionate supporter and defender of Dartmoor.

And he is not even the most famous person in the family. That honour belongs to his aunt, Angela Mortimer, who won the ladies' singles at Wimbledon in 1961.

With such role models it is little wonder that Ian felt the pressure to succeed at an early age.

He says: "What my aunt did was a big guiding light, it's such an amazing achievement. One of things growing up with a proven standard of success like that is that you've got to be world-beating.

"I'm very, very lucky in that I've always known I was quite clever, and cleverer than most people. Always known it's possible to achieve and had the possibility to write something and always known a target for success."

Although Ian was born in Kent and educated at Eastbourne College, Devon was always the family home.

His forebears ran one of the largest companies in the county, Mortimer's Cleaning and Dyeing, and he can trace his roots to parishes around Dartmoor and the Teign Valley.

"They had been here for 400 to 500 years. In fact, I was the first member of the family to be born outside Devon.

"Growing up in Devon with a name like Mortimer was part of the story. Every piece of furniture wasn't just a piece of furniture — there was a story attached to it.

"The chair I normally sit on, for example, was once owned by the Bishop of Exeter."

As a boy he would visit castles and learned to see history in all of his surroundings. Now, when he walks around Moretonhampstead, he sees the past and its people in the land.

"I read the landscape, look at the lines," he says. "I'm the chairman of the local history society and when I walk around the parish I get up high and see the troughs and flats.

"I don't relax like most people do, but I don't work like most people do either."

His approach to writing is like a medieval knight storming the barricades. Glass of whisky in hand, flanked by meticulous research notes, he attacks the page from morning till midnight, stirring only for that daily walk and to share meals with his wife and children.

"When I'm working around the clock it is something special, a sort of magic, and I can't take my mind off it," he says.

"I wake up two or three times a night with ideas. It is everywhere and I'm in a trance-like state."

He operates an open door policy for the youngsters, two boys and a girl aged seven to 11, who can watch him write at any time they please.

He met Sophie in 1994 after a mutual friend suggested that she might like the idea for a book he had. The idea would become the Time Traveller's Guide, which was written in 2007.

After they married she encouraged him to take the plunge and become a writer and in 2001 he wrote the life of Roger Mortimer. It was followed by The Perfect King: The Life of Edward III.

He admits writing the first book was 'really, really difficult financially'. For the second book he borrowed £20,000 and wrote it in exactly a year, thanks to 90-hour-a-week stints at the keyboard.

He says the support of Sophie was crucial, as normal family perks and holidays were put on hold in the belief that Ian would become a success.

Ian's other works include: The Fears of Henry IV; 1415: Henry V's Year of Glory; numerous book reviews and magazine articles; notes and essays on historical figures and the nature of history itself.

The book on Mortimer, a ruthless one-time ruler of England, stirred much critical debate.

The book pioneered the idea that Richard II had not died in Berkeley Castle in 1327. Some scholars who reviewed the book did not agree.

"I don't take criticism well," Ian admits. "Especially when it comes from people thinking their opinion is more important than my scientific theory."

He sits on the Dartmoor National Park Authority.

"I'm about making people realise the value of it," he says. "It is a fantastic treasure. It is the largest open area in southern England, with the largest collection of Bronze Age remains in Europe.

"I want to make people realise the value of what they have."

His success has made him realise the value of what is around him and how it can be lost.

His father, John Mortimer, an architect from Plymouth, died from the inherited Fabry's Disease at the age of 58.

"I was very close to my father," he says. "It is 17 years since he died, but I still think about him. He was told he had six months to live and lived for another 15 years."

He says the experience of that and other members of his family have taught him to 'get as much out of life as I can'. Thankfully, he is free of the disease.

He is working on The Roots of Betrayal, a sequel to his first novel, Sacred Treason. The latter will be published on August 5.

Both are written by Ian using his middle names, James Forrester.