The politician Earl Grey is one of a handful of English aristocrats – others include the Earl of Sandwich, the Earl of Cardigan and the Duke of Wellington – to have his name immortalised through association with an item familiar in most households.

The product in his case is a special kind of tea, flavoured with bergamot, which also lends its exotic scent to eau de cologne. Charles Grey received a gift of some, probably as a diplomatic perquisite, then caused more to be manufactured for sale.

Besides this claim to fame, Grey demands historical attention as the prime minister whose government passed the 1832 Reform Bill and saw in the abolition of slavery across the British Empire the following year.

His work as an abolitionist was occupying the attentions of a young Oxford academic, Amanda Foreman of Lady Margaret Hall, as a subject for her doctoral studies back in the 1990s

Her researches led her to a box of letters to his one-time mistress, Georgiana, Duchess of Devonshire – and to an entirely different subject for her thesis.

“I knew by the end of the first letter – what an amazing woman she clearly was! – that here was the subject for me,” she told an audience at the Buxton Festival at the weekend.

The former Lady Georgiana Spencer – great-great-great-great aunt of Diana, Princess of Wales – was a fully liberated woman long before the term was known.

The scandalous affair with Grey, with whom she had an illegitimate daughter, was not the half of it. She also carried on a lesbian affair with Lady Bess Foster, who was simultaneously sharing the bed of the Duke of Devonshire, observably a menage à trois that was the wonder of society.

He was the country’s richest man but, even so, still shuddered at the debts run up through his wife’s addiction to gambling.

All this will be known to many of my readers, for Dr Foreman’s thesis was the basis of her best-selling biography Georgiana, Duchess of Devonshire, published 20 years ago and still selling in shedloads.

The book in turn led to a Channel Four docudrama about her, a Radio 4 play starring Judi Dench and a big-budget movie, The Duchess, with Keira Knightley in the title role and Ralph Fiennes as the duke

Now we have an opera on the subject – brilliantly conceived and executed – which this week delighted its first audiences at Buxton Opera House, as part of Buxton International Festival.

The venue could hardly be more fitting since it was entirely due to Georgiana’s husband, of nearby Chatsworth House, that Buxton was developed as a spa town.

He paid for the remarkable buildings that grace the town, among them the glorious Crescent, by the York architect John Carr. This is undergoing a long and costly renovation under the developer Trevor Osborne, who had charge of Oxford’s Castle site revamp. He it was who introduced me a decade ago to the delights of Buxton’s annual binge of culture – now celebrating its 40th anniversary – since when I have visited every year.

The opera Georgiana proved a rich reward this year, not least for the well-managed ‘book’ (to borrow a term from the world of musicals) by Janet Plater and the exceptionally witty lyrics from Michael Williams, the festival’s chief executive officer.

The music for this ‘pasticcio’ was sourced by Mark Tatlow, from a number of composers, principally Mozart. The choice was from his lesser -known works, including the operas Il re pastore and (in good measure) La finta giardiniera, both of which I first heard at Garsington.

The singing throughout was top notch, and especially by Samantha Clarke as Georgiana, Susanna Fairbairn as Bess and Benjamin Hulett as the duke.

An adroit touch was the commentary on the action , supplied in stand-up comic fashion by the playwright Richard Brinsley Sheridan (Geoffrey Dolton) and the Whig politician Charles James Fox (Aled Hall).

Buxton’s opera programme began with a fine home-grown production (under conductor Adrian Kelly) of Tchaikovsky’s Eugene Onegin, notable for an affecting Tatyana from Shelley Jackson.

The 200th anniversary of the birth of Jacques Offenbach is marked in the knockabout fun (compete with can-can) of his first big success, Orpheus in the Underworld.

The opera rarity, a Buxton trademark, comes in Lucio Papiro Dittatore, by Antonio Caldara.

Oxford writers are well represented in the festival’s literary thread, among them the novelist Tim Pears, there tomorrow to discuss The Redeemed, the last part of his West Country Trilogy, and Sir Diarmaid MacCullogh, Oxford University’s Professor of the History of the Church, on his biography of Thomas Cromwell.

The festival continues in the lovely spa town until July 21 (