"She" was Margaret, Duchess of Argyll, socialite, superstar, dame fatale, for whom the maxim "Go to bed early, and often" proved all too consuming. She made her first entrance as a debutante, contriving to spill red wine down the customary white gown so as to enable her to don the ice-blue confection hanging idly in her wardrobe. As Mrs Charles Sweeney (her first husband), she was immortalised in Cole Porter's "You're the Tops". Her subsequent marriage to Ian Campbell, Duke of Argyll (Scarlet O'Hara and Rhett Butler had nothing on this match), ended in the longest and most notorious divorce case in English legal history. Bored by Campbell's reluctance to play the social circuit with her, she had retreated into a secret life of anonymous escorts and liaisons in hotel bedrooms. She became a kind of Donna Giovanni. There were photographs (though nobody knows how), and one, which became known as "the headless man" (for obvious reasons: the gentleman was standing, the Duchess was not), found its way into court, prompting the Judge into "operatic" paroxysms.
At least they promise to be so when Philip Hensher's "racy libretto" meets Thomas Ades's "amazing" score in tomorrow night's Cheltenham premiere. Other aspects of this extraordinary story are stranger than fiction, but the more Jill Gomez has unearthed in her research (and she's not about to reveal how intensive her methodology has been!), the more she has found herself able to identify with a woman who at first appeared so irretrievably shallow. It's the absolute strength of purpose that has fascinated her; the command of the woman, her dignity through the ultimate vilification.
"Had she lived in today's world, she'd probably have been chairperson of Save the Children or something... But, you know, the opera is not really about her. It's about the myth she became. She herself speaks of outlasting fashion, outlasting time..."
A metaphor, then, for the changing face of society? (Remember, the Duchess came to notoriety on the threshold of the swinging Sixties.) History as maker of myths? "That's all part of it. It's about endings and beginnings, it's about the inability to recognise things changing, the inability to let go of the past and embrace the future..."
The composer, Thomas Ades, has no such problems. His music plainly has a past - pieces like Living Toys have absorbed much but emulate nothing - but its future is uncharted. You know where it's coming from, but where it goes... chances are it's somewhere you've never been before, or don't recognise. And that's how Jill Gomez would sum up the score of Powder Her Face.
"Each of the eight scenes has a totally distinct musical personality; and, in a funny kind of way, it's only when you get to the end that you understand what he was doing at the beginning... When I started work on this piece, I felt very strongly that I had a past, that I had ancestors that I could think about and dream about. Tom's music does that."
But - and this is the burning question - is it singable? Does it alleviate fears that composers no longer understand the voice? Is it grateful for the voice? "Is it grateful!" - and here her intonation suggests an intoxicating mix of excitement and apprehension - "Oh, yes, and extraordinary. Just the sheer range of it - bottom G up to luminescent top B, full forte like Salome! It's a whole gallery of operatic heroines, but within the tighter form of a chamber opera, moving swiftly from scene to scene as in a song recital. This Duchess goes from a girl in her twenties to a grande dame in her late seventies, from a kind of Isolde-like rapture to Cole Porter insouciance - yes, I can promise you an Ades popular song which will stay with you long after the opera is over - and then I'm into an Ella Fitzgerald scat-style, and from that to the fragile hauteur of a Richard Strauss heroine, and ultimately to a manic Erwartung-like psychodrama... Vocally, physically, mentally, it's more demanding than anything I've ever done... You remember how Robert Craft described the Erwartung lady - 'an Isolde with a nervous breakdown' - well, when this is over, I'll probably feel more like Janacek's 300-year-old Emilia Marty in need of a somewhat overdue break."
But still there's the overwhelming excitement of coming fresh to a new piece, the feeling that it is somehow evolving with you (in this case, many of Ades's refinements were specifically tailored for the singer). "It's scary but liberating coming to something with no tradition of performance to put behind you. But then I've tried to approach everything I've ever done in that way, to look at the music as though I was the very first person ever to see it."
Initially, it's a technical process. Gomez learnt Powder Her Face from the inside out. Scene 4 - that's the "headless man" scene - came first. Because of the difficulties, because it's so "off-the-wall".
"This scene takes off musically unlike anything I've ever heard. We've seen nudity, we've seen all kinds of sexual practices on stage - but we've never heard them before!" Enough said.
Gomez is no stranger to controversy on the operatic stage. In creating the role of Flora in Tippett's The Knot Garden, she went topless at the Royal Opera House and, to this day, will never forget the unmistakable sound of coins dropping into the slots for opera glasses. That was in 1970, when hers was the name on everybody's lips (roles like the Countess in Figaro, Tatyana in Onegin, Cleopatra in Giulio Cesare and the Governess in The Turn of the Screw duly reaffirmed that early promise).
Aficionados who know and respect her as the thoroughbred artist she is, will ask why, in recent years, she has all but vanished from the operatic stage. Apart from one brief "crisis" of confidence back in 1975 (the knock- on effect of singing through a chest infection), there is no straight answer. Ask the opera houses.
Does she care too much? As one who sat by her through a whole week of the 1991 Cardiff Singer of the World Competition, I know her as someone with very real concerns for the craft, for style, for language, for vocal truth, someone who feels for young singers who compromise their voices in pursuit of the glittering prizes. She has scruples. Maybe they have cost her.
But, in another sense, the theatre lives in everything she does - not least in her ambitious recital work. She comes to the very first line of this role through the experience of songs like Hugo Wolf's Das verlassene Magdlein. "Wolf's forlorn maiden is right there. In one bar of music, those with ears to hear will hear the history of everyone who has ever felt betrayed and alone."
For her, it's a moment, a role, that's been waiting to happen. A little more powder, a touch of lipstick, and she'll be ready.
n 'Powder Her Face' is at the Cheltenham Festival, 8pm tomorrow, Everyman Theatre (booking: 01242 227979), then at the Almeida Theatre, London N1 on 5, 9, 14, 17, 22 July (0171-359 4404)