Paul Doherty wants to dig up a grave. Not just any old grave: he wants to open up the marble tomb of King Edward II in Gloucester cathedral and test the DNA of the 700-year-old bones inside - because he believes they belong to the body of an impostor.

The mystery surrounding the death of Edward II - reputedly skewered with a red-hot poker by order of his wife, Queen Isabella - has intrigued Dr Doherty ever since he turned away from academic life (he did a PhD in medieval history at Oxford in the early 1970s) to become a history teacher.

No one saw Edward’s body close up, he argues; no one spoke to the mysterious old woman who laid him out; the man supposedly guarding him did not even know he was dead. Someone could easily have spirited him away to France and handed the undertakers the corpse of a lookalike.

The theory seems both credible and incredible simultaneously - exactly the kind of tale that appeals to Dr Doherty, who has written 62 novels (historical mysteries and romances) since he became headteacher of Trinity Catholic high school in Woodford Green, Essex, in 1981.

If you ask Paul Doherty to list the titles of his books, they come as a bit of a gory surprise: Satan’s Fire, Corpse Candle, Murder most Holy, The Field of Blood. But then ask him what’s going through his mind and that’s no less surprising. “During lunch at school today I was eating a beef pie and it started me off thinking about poisons. Did you know that in Venice, to test a man’s innocence, they used the abrin (Rosary pea) seed? If you were innocent nothing was supposed to happen when you ate it; if you were guilty, you died. The seed is poisonous if it’s chewed, but if you swallow it without chewing, it’s harmless: it goes straight through.

“I was wondering how to use that in a plot. In the hurly-burly of school I find it helps me relax if I change what I am thinking about. Though if someone asks me what I’m thinking about, I say the building programme. By the way, the pie was delicious.”

Academia was his second abandoned career - from the age of 11 to 19 he went to a Catholic boarding seminary, destined to become a priest. Each brought him a store of quirky expertise: in the seminary he studied ancient Egyptian scriptures; at Oxford his thesis was on Edward II and Isabella. So although he cycles off to the Public Record Office or the London Library for factual research, the background canvas for his books - the food, the clothes, the rituals - already exists before he embarks on the plot.

As a history teacher, first in a girls’ independent school in Ascot, then in comprehensives in Newark and Surrey, he was always interested in storytelling: putting flesh on the bones of history (and sometimes taking it off again). But it was while rocking the first of his seven children as a sleepless infant that he began to plan novel writing. “I used to think ‘Go to sleep’ and I was writing in my head.”

It took several years before a publisher accepted his first book, The Death of a King, about Edward II; then, with big sales in the United States in particular, the company began to demand more. These days he writes two or three a year, at his desk at home in the early mornings and late at night.

He tries out ideas with fund-raising lectures at Trinity, a beacon school and science college with an outstanding Ofsted report in April 2000.

Parents, students and local people come to hear him speak on ghosts and smugglers, Alexander the Great’s murderous mother, the true identity of the man in the iron mask. “It’s wrong to start speculating that the ancient Egyptians were Martians, but there has always been this undergrowth to historical fact where things might have been, could have been.”

What might have been is true for him and his students, too. One of a large family, son of an Irish immigrant factory worker, he failed the 11-plus in Middlesbrough and, if not for the seminary, would have gone down the docks or the pit. Such alternative histories inspire him as a head, a teacher and a writer. “I listen to people at school, the way they talk, the situations they are in. People have strengths and depths: when they are confronted or in difficulties they can show a courage or a ruthlessness that is quite breathtaking.”

Henry VIII was violent and sexually inadequate; Alexander the Great combined brilliant strategy with panic attacks. And the sex and violence?

“That’s life. That’s history. My books are about restoring order and harmony; about the pursuit of evil and the vindication of good. I see that in students, too. I go home to write and I take my school with me. I come in here and bring the novels with me. I can’t divide up parts of myself.

That’s the way I am.”

Isabella and the Strange Death of Edward II by Paul Doherty will be published by Constable Robinson on February 27, pound;18.99