First it was Richard III, whose reputation as paranoid and a child murderer has lately been revised in the light of new evidence.

It's as if a picture restorer had been set to work on his blackened image, rubbing away the grime and misconceptions of centuries to reveal instead a sensitive, even likeable man, who should be pitied rather than reviled.

Now it's the turn of Bess of Hardwick. The second wealthiest woman in England in the time of Elizabeth I, from whom our present queen is descended, Bess was for many years the jailer of Mary, Queen of Scots. According to an extensive cache of letters discovered by academics at the University of Glasgow, the Countess of Shrewsbury was not the calculating, mercenary, ambitious termagant of popular myth, but instead a clever, astute and motherly woman, who even showed a smidgen of affection for her four spouses.

It's good to see a little nuance added to the portrait of a wife whose influential position and enormous fortune would have made her the object of envy and bitterness. Such women rarely get a fair press, now as then.

Yet Bess's true claim to fame, and the reason these freshly discovered letters are so tantalising, is her unique relationship with Scotland's tragic queen. As wife of Mary's captor, the hen-pecked Earl of Shrewsbury, she became one of the most significant figures in this melancholy monarch's life. Apparently they stitched tapestry together to pass the time. The earl was not so under his spouse's thumb, however, that he could not stir her jealousy, and Bess openly accused him of having an affair with Mary. Despite their companionable needlework, this does not suggest a warm rapport between the younger queen and her landlady.

One feels sorry for Mary – or at least I do. It is astonishing, though, that more than 400 years after her execution, many continue passionately to mistrust and loathe her. In fact, since the day she went to the block, only one thing about her has remained constant: the intense speculation she inspires.

It's said that, with the exception of Jesus, no-one has had more books written about them. Mary Stuart is to historical inquiry what Princess Diana was to the tabloids: a source of unending gossip and interest, much of it salacious and sexist, some of it puzzled, and all of it doomed, one feels sure, never to be fully explained.

Like Diana, Mary's allure lies partly in her grisly death. Yet it was the brilliance of her younger life, and her ultimately fatal political and sexual naivety that she is best remembered for. It's not the headless corpse we dwell on, but the auburn beauty, an exotic the likes of which Scotland – indeed Britain – had never seen before. Glamorous and intellectual, she behaved as if the normal rules of conduct did not apply to her. Whether one blames that on her cosseted French upbringing and the shock of arriving in a rough and rebellious country, or whether you attribute it to a temperament that was high-handed, selfish and impetuous, is a question of taste. Both interpretations, probably, are fairly accurate.

Were Mary alive today, she would be roasted by the red-tops, as she was by John Knox and his cronies. But at least she would have the right to a proper trial, and access to a crack team of defence lawyers who might be able to prove she was the victim of a conspiracy, rather than the murderous author of her own downfall. Whatever the verdict, she would certainly be spared the beheading which reduced even supposed enemies such as the Earl of Shrewsbury – but probably not his wife – to tears.

Whatever the truth of Mary, it is the mystery and ambiguity that surround her motives and behaviour that have kept her flame alive. Baffled and intrigued, historians return again and again to reassess the facts. I'm not sure, though, how we would feel if a hoard of documents was discovered that proved exactly what she was like. Do we really want a clearer picture of her? Personally I prefer the canvas in its old, cracked state, half in shadow and leaving everything in doubt, except our fascination.