What were Macbeth and his wife Gruoch really like?

• Macbeth gets his comeuppance in an Edinburgh Festival performance. Picture: Neil Hanna

From Elena Ceausescu, wife of the Romanian dictator, to Mira Markovic, widow of the Serbian demagogue Slobodan Milosevic, not to mention convicted serial killer Rosemary West, each has been nicknamed a "Lady Macbeth" in tabloid-speak.

Scottish historian, writer and broadcaster Fiona Watson sighs wearily as we discuss the Macbeths' dark, demonic reputation. Thanks to William Shakespeare, she acknowledges, there are few places in the world where the name Macbeth is unknown, and few societies untouched at some point in their histories by the kind of reign of terror over which he supposedly presided.

Watson has written a thrilling new "factional" biography of Macbeth, and points out that, for some, the fact that there was actually a Scottish king called Macbeth – who lived and died 600 years before he was immortalised, some would say traduced, by Shakespeare – will come as a surprise.

Watson believes that Shakespeare's portrait of the terrible, corrosive consequences of the reckless pursuit of power has had a powerful effect on the collective psyche precisely because it was based on the life of a real man who once stood in places that we can still visit.

These include the blasted heath near Nairn where Macbeth and Banquo meet the three weird sisters, the castle in Inverness – "or its descendant" – where Duncan was murdered, and Birnam wood, whose foliage provided cover for Malcolm's army, and Dunsinane, site of Macbeth's denouement.

"As with the portrayal of the Scottish hero, William Wallace, in the 1996 film Braveheart, the factual basis of Shakespeare's story gives added authority to the universal truths with which the play deals," writes Watson.

Now 44, Edinburgh-born Watson was senior lecturer in Scottish history and director of the Centre for Environmental History at Stirling University until she gave up academia to write and broadcast full-time. She presents History File on BBC Radio Scotland and fronted BBC2's 2001 ten-part series In Search of Scotland.

"Now, I believe that Macbeth's moment has come," she says, explaining that the real king, who ruled Scotland for 17 years from 1040, may have been demonised in death, but in life, like Shakespeare's king, he "bought golden opinions from all sorts of people".

He may have murdered his way to the throne, killing the king, Duncan – who, Watson says, was definitely not the gentle old man portrayed by Shakespeare – but Macbeth brought peace to Scotland in violent times. He was an effective and popular ruler and the first Scottish monarch known to have made a pilgrimage to Rome.

Therefore, says Watson, it is difficult to exaggerate how great an injustice history has inflicted on Macbeth and his wife, Gruoch.

Still, with the Royal Shakespeare Company about to stage a new play, Dunsinane, by acclaimed award-winning Scottish playwright David Greig – which asks what happened after Macbeth was overthrown and what became of "the tyrant's widow" (who is being played by Scottish actress Siobhan Redmond) – perhaps Watson still has a fight on her hands in order to redeem the Scottish king's tattered reputation?

"I know, I know," she replies, with a resigned laugh. In fact, she was consulted by Greig, over a long lunch, when he was researching his play, since she's such an esteemed authority on Scottish history. Indeed, you could say Watson is as steeped in the Dark Ages as Shakespeare's Macbeth was steeped in blood, although she's currently writing a novel set in 18th-century Scotland.

She grew up in Dunfermline, after her parents moved from Edinburgh when she was barely 18 months old.

A voracious reader since childhood, she was educated at Pitreavie Primary School and Dunfermline High, where she was in the top stream ("and got some stick of it!"), before reading medieval history at St Andrews and she then gained a PhD in Scottish history at Glasgow University.

She says she can't remember a time in her life when she wasn't enthralled by history.

"Even when I was quite wee I loved reading. There was something wonderful about escaping into other times and other worlds," she says, adding that Mary, Queen of Scots was one of her earliest obsessions.

She's had a fondness for Shakespeare's "Scottish play" since studying it at high school, where she had an inspirational English teacher.

"I've never been quite sure why the past has such an exotic appeal for me. Obviously, I grew up with Robert the Bruce, above the abbey in Dunfermline, but then not everyone who grows up there is similarly moved!"

She now lives in the town of Braco, in Perthshire, with her husband, Nick Hanley, professor of environmental economy at Stirling University. They have one son, Finn, six, and Watson is step-mother to Hanley's grown-up son and daughter from a previous marriage.

"I'm afraid I'm still living quite happily in the past, though," she jokes. "Especially the deeper past, where you don't have to swim through a morass of information."

Which brings us back to Macbeth, about whom there is a dearth of factual information. Details about his life and times do exist in contemporary chronicles, Watson says. You just have to know where to look. And, given Scotland's close connection with Ireland in that period, she also turned detective and discovered at least as many illuminating snippets of information about what was happening in the kingdom of Scotland in Irish sources as in the indigenous Scottish ones.

However, piecing together the life of a medieval king, about whom there is very little evidence, was not an easy task, so Watson has taken the bold step of fleshing out his story with some fictional passages, hence the "factional" nature of Macbeth: A True Story.

"These are clearly marked in italics in the book so I hope they'll be taken in the spirit in which they're written, as pointers towards potential insights and realities," she says. "They are there for the reader's entertainment and edification."

She also wanted to show that long before Shakespeare's imagination turned this noble Scottish king into one of the most resonant names in history, immediately after his death in 1057, the medieval spin-doctors began trashing his character and reputation.

"The true story of Macbeth opens a window on the so-called Dark Ages. It was a complex time in European history, which has been largely misunderstood, and I hope my book also opens a window on Scotland, which for too long has been seen as a poor benighted historical subject."

It is far too tempting these days to interpret the evolution of Macbeth's posthumous reputation as a terrible Anglo-Scottish conspiracy, she says, "the mighty Celtic king succumbing ultimately to those who had neither sympathy for nor understanding of an ancient and honourable way of life".

"We will have him turning blue and crying 'Freedom' soon enough if we go down that route," she warns. "Romanticising the Celt is just as patronising and derogatory as outright insult."

She's impatient with the victimhood notion of much Scottish history and the idea that Celtic Scotland was some kind of idyllic, isolated and time-proof Brigadoon.

But perhaps history's real victim has been Lady Macbeth? A royal princess in her own right, she was the grieving widow of Macbeth's cousin Gillacomgain, whom Macbeth had slaughtered, and therefore the mother of a fatherless son, Lulach, when she married her first husband's murderer. Perhaps she, too, should be reclaimed by Watson?

"There's no doubt Gruoch has perhaps been wronged by history even more terribly than Macbeth himself," she agrees. "Medieval women may be more or less silent to us, but I believe this doubly royal woman played an active role in both in her marriage and in public life generally.

"Remember, she made a political match with Macbeth: what she and her son needed was a strong protector. In the circumstances, Macbeth fitted the bill perfectly."

Finally, if Hollywood were to come calling with a request to buy the rights to film Macbeth's true story, who would she wish to play the great warrior who, she's discovered, was tall, fair, and ruddy-cheeked? (In her book, the only theatrical Macbeth pictured is Orson Welles's moody monarch in his movie version of the play.)

She hesitates for a moment, gazing thoughtfully into her coffee cup, then says: "I'm afraid the actor who I think would have played him best is dead: Patrick McGoohan (star of the cult series The Prisoner]. All the time I was writing about Macbeth that's who I pictured and saw in my mind's eye."

Ah well, just so long as it wasn't Mel Gibson.

• Macbeth: A True Story by Fiona Watson (Quercus, 20).

Dunsinane opens at the Hampstead Theatre, London, on 10 February.