There is a border between the lands of myth and fact which, author and historian Alison Weir told me, you cross “at your peril”.

She should know. She has now written two books about Eleanor of Aquitaine (1122-1204). The first was a history, Eleanor of Aquitaine, sticking closely to facts and examining all sources rigorously.

Now she has written The Captive Queen, in which she allows myth, legend and conjecture to creep in — though (and this is the important point) always making it clear in notes at the end of the book when that has been allowed to occur.

Eleanor was that remarkable queen, who, “by reason of her excessive beauty, destroyed or injured nations” according to one contemporary chronicler. She married first Louis VII of France and then Henry II of England (1133-1189). She was the mother of 12 children and co-ruler of a realm stretching from Northumberland to the Pyrenees; but she has been remorselessly vilified by myth and legend over the centuries — not least by allegations that she murdered Rosamund de Clifford at Woodstock.

The author, who will this week talk at Woodstock Town Hall as part of the Woodstock at 900 festival, said: “I am a popular historian who would have been frowned upon a generation or two ago.

“History has some great stories to tell, but you have to piece them together carefully; you cannot just let rip your imagination; you must keep to the constraints. Readers care passionately about accuracy, and whenever you depart from history into myth or legend you must justify it.”

The story of Eleanor is certainly a rattling good one. Several of her children, including the future kings Richard and John, were born at Beaumont Palace, Oxford, which stood opposite the present Worcester College at the end of Beaumont Street. And of course there was Woodstock Palace, built by Henry I (her second husband's grandfather) and the supposed scene of Eleanor’s murder of Fair Rosamund, otherwise known as Rosamundi, or Rose of the World.

Disappointingly, perhaps, for those of us who never like the facts to get in the way of a good story, Ms Weir told me: “There is no evidence at all that Eleanor visited Woodstock on her journey through Oxfordshire of December 1166, though some historians have decided she must have done so.

“In fact there is no contemporary evidence linking Rosamund de Clifford with the palace or hunting lodge at Woodstock, nor is there any proof that a bower or labyrinth ever existed there.”

In legends, of course, Henry II constructed a labyrinth or maze at Woodstock to hide Rosamund from the jealous Eleanor — to no avail, since Eleanor followed a thread of silk from her dress and found her. But although Henry did indeed construct a cloistered garden at Everswell, near what is now called Rosamund’s Well in Blenheim Park, there is no evidence to link it with Rosamund. All the same, in history, Henry II did indeed have a beautifully decorated tomb constructed for Rosamund at Godstow Nunnery, Oxford, where she died in 1176 or 7.

In the 14th century, the French Chronicle of London gave a lurid account of how Eleanor murdered Rosamund at Woodstock in a particularly unpleasant fashion: she stripped her naked and roasted her between two fires and, with venomous toads between her breasts, let her bleed to death.

Alison Weir, who lives in Surrey but often tours Oxfordshire to discover scenes of her subjects, is probably now our most popular historian (her first book on Eleanor has sold more than 150,000 copies). But how did she get into the business in the first place, and how does she find time for her prodigious output?

She said: “At the age of 14, I was so enthralled by a lurid novel about Katherine of Aragon that I dashed off to read real history books about what I had read.

“By the time I was 15 I had written a three-volume reference work on the Tudor dynasty.”

Before becoming a full-time writer, she was a civil servant, writing as a hobby. A generation or two ago, few historians were content to be merely storytellers, but the wheel of fortune has turned.

“Anyone can teach themselves history,” said Ms Weir.

And if any academic is sniffy about that, there is always Shakespeare: he certainly wrote “faction”.

The Woodstock at 900 lecture is on Tuesday, May 11, at 8pm (see Bookings, right).

Eleanor of Aquitaine, By Wrath of God, Queen of England is published by Vintage at £9.99. The Captive Queen is published by Hutchinson at £14.99.