A magnificent portrait of Maria Countess Waldegrave by Sir Joshua Reynolds in the Dunedin Public Art Gallery has significant links with the royal engagement, because Maria was Prince William's sixth great-grandmother.
In her lifetime (1736-1807) she was snubbed by the Royal Family and slandered for her low parentage, but her daughters, whom Reynolds famously painted as "The Ladies Waldegrave" (1780), were much sought-after as brides.
In 1786, her third daughter, Lady Anna Horatia Waldegrave (1762-1801), married into the Seymour family, ancestors of Lady Diana Spencer and Prince William.
Maria, Countess of Waldegrave and Duchess of Gloucester and Edinburgh, has had her revenge.
Maria Walpole was the illegitimate daughter of Sir Edward Walpole, brother to Horace Walpole, who led Parliament between 1720 and 1740 in what would become a prime ministerial role.
The Dictionary of National Biography reports that Maria, the daughter of a postman, was discovered sitting on a dustcart by Catherine, wife of the clergyman Thomas Secker, who declared that she never saw a more lovely creature.
Mary Kisler reports in her recent Angels and Aristocrats: Early European Art in New Zealand Public Collections, that many were envious of her marriage in 1759 to James, second Earl of Waldegrave.
Lady Mary Koke wrote that while Lady Waldegrave was a lovely woman, she had little sense, even if blameless in character and conduct.
But, she continued, there was no disguising the fact her mother had kept "some infamous house", and "from the top of a cinder cart", she had used her beauty to lure Sir Edward Walpole.
None of that was true. Maria found herself a widow only four years later.
"As she is so young," wrote Horace Walpole, "she might find as great a match and a younger lover."
She did indeed.
The Reynolds portrait, painted in 1762, just before the earl's death, displays the attractions that led the Duke of Gloucester to begin his ardent pursuit of her in 1764.
Maria was claimed to be the most handsome woman in Britain, says Kisler.
But unlike Lady Mary Koke, she observes that in the Reynolds portrait Maria appears more intelligent than classically beautiful.
Kisler speculates that portrait's low viewpoint shows it was meant to be hung high on a wall, so that she sails above us like the prow of a ship.
Her garment, she says, suggests she is a kind of latter-day Winger Victory of Samothrace, a magnificent trophy for any spouse.
The duke obviously thought so too, for as the DNB remarks, Maria exceeded her uncle's expectations about finding a great match.
Maria and the Duke of Gloucester were secretly married on September 6, 1766.
Over the subsequent few years, Maria claimed as many royal privileges as she could, accompanying Gloucester on social occasions and dressing her servants in approximations of royal livery.
George III, Gloucester's brother, was unhappy that he had a mistress, and sent the duke on a series of diplomatic missions abroad.
Maria's pregnancy in 1772 forced Gloucester to reveal the marriage to George III, who immediately undertook an inquiry into its validity to ensure the legitimacy of the child.
The king was relatively sympathetic to Gloucester, but the duke, duchess and their children were barred from the royal presence and Gloucester's missions were stopped.
The Gloucesters struggled to maintain the trappings of royal status and a growing family on his existing settlement of 29,000.
A financial crisis of a kind common to eighteenth-century royal dukes made the family flee to Italy.
In the early 1780s, the duke began a sexual relationship with his wife's lady-in-writing, Lady Almeria Carpenter, described as one of the most beautiful women of her time, but to whom nature had been sparing of intellectual attractions.
Gloucester, the duchess and the lady-in-waiting continued to travel and maintain a joint household, though Carpenter was never allowed completely to eclipse Maria.
In 1772, Maria's marriage to the Duke of Gloucester, together with the Duke of Cumberland's to another commoner, Mrs Anne Horton, forced George III to institute the Royal Marriages Act.
It ordered all royal offspring to receive permission from the king before marrying.
Even now, it provides stringent safeguards against undesirable marriages that might affect the succession to the throne or lower the status of the Royal House.
Edward VII, for instance, was only able to marry Wallis Simpson after his abdication.
A Bill to repeal the Act was dropped in March 2009. Maria lived from 1736 to 1807.
After Gloucester's death, she moved to the Brompton Rd, London, and was buried at St George's Chapel, Windsor.
Jocelyn Harris is emeritus professor in the English department at the University of Otago.
