By Julie Ann Godson (Harris Manchester, 1997)

At dusk on a snowy March evening in 1766, a tired young couple made out the welcoming lights burning in the windows of the creaky old manor house that was to be their home. He was William Flower, 2nd Viscount Ashbrook, 3rd Baron Castle Durrow in the County of Kilkenny, she was Betty Ridge, daughter of a humble Thames fisherman. Earlier that day they had been married in a little Oxfordshire church, and now Betty was embarking on a new life in the alien world of the aristocracy. 

No, I didn't believe it either when I first heard the story four years ago. But I'm supposed to be an Oxford-trained historian, so I thought I'd better check it out. If I dismissed it as a fairytale and it turned out to be true, I would look very silly indeed. 

At the time, speculation was rife about the possibility of a prince marrying a commoner but, unlike today, in the 18th century such a step would not have been commended as laudably egalitarian. Aristocratic marriages were designed to reinforce the interests of a closed shop of élite landed families, not to fulfil romantic inclinations. How could a mere fishergirl who brought no dowry with her possibly enhance Flower-family interests?

As it turned out, the challenges Betty would face proved far greater than she could have imagined. Following an idyllic 14 years of marriage, she was widowed. Her son, the heir to the Ashbrook title, was a boy of just 13, and her husband had no other male relatives living on the Ashbrook side. But Betty had inherited the characteristic Ridge-family determination which revealed itself through the documents I used to piece together the remarkable story of her own parents, Ridge cousins. Luckily, so had her elder brother William, another fisherman. William Ridge was despatched to Ireland where he embarked on a sharp learning-curve, taking control of the Ashbrook estates and running them on behalf of his sister and nephews for a quarter of a century. 

Sensible of the resentment that had built up against the Ashbrooks as absentee-landlords, William persuaded Betty to bring the young heir over to Ireland to meet his tenantry. It was a brilliantly-calculated move. The sight of the pretty young widow and her son accepting the salute of the Durrow militia defused local hostility and ushered in a period of mutual sympathy which lasted even into famine times in the following century. As Anglo-Irish proprietors go, the Ashbrooks are remembered fondly in Durrow to this day. Is it too much to suggest that the two children of an Oxfordshire fisherman brought a sympathy to their dealings with the rural poor with which most Irish tenants were not familiar?

The family fortunes had taken a knock when the 2nd Viscount married a girl who brought no dowry with her. The couple's elder son appears always to have been frail. In spite of fitting out Wadley House near Faringdon as a prospective marital home, he died a bachelor at 35. It was up to Betty's younger son Henry to swoop to the rescue.

The dashing soldier rushed back from fighting the French in Egypt and, within six months of becoming 4th Viscount, he married a rich heiress from Woodstock. More Ridge than Ashbrook, he set about restoring the Ashbrook fortune and family prestige, purchasing a flashy mansion on the bounds of Windsor Great Park. He struck up a life-long friendship with the Duke of Clarence who so unexpectedly became King William IV, the 'Sailor King'.

It was a project which culminated in the most glittering marriage in the entire history of the Ashbrook family when Henry's daughter, granddaughter of a fishergirl, became Duchess of Marlborough. From the banks of the Thames to Blenheim Palace in 80 years. Not bad. And, as so often is the case, the reality is far more intriguing than the myth.

The Water Gypsy: how a Thames fishergirl became a viscountess by Julie Ann Godson is available now for £9.99.