Soprano Isobel Buchanan is wagging a finger at me intently from across the kitchen table.

"I don't care how much anyone tells you about technique," she says. "Singing is all about the mind. The minute your confidence goes, everything else starts to fall apart too."

Buchanan has experienced it both ways. Hers was a stratospheric early career: in the 1970s she was Scotland's golden operatic talent, swept off to Australia by no less a figure than Joan Sutherland and catapulted into star roles and a staggeringly young international career. But an undiagnosed physical conduction left her confidence dented and she withdrew from the limelight. Now 61, she has recently returned to the stage thanks to proper medical care and recital formats in which she feels comfortable again. When she sings at the St Magnus Festival in Orkney this summer, it will be her first appearance in Scotland for decades.

Buchanan was always a natural singer. Raised in Cumbernauld in a house "where nobody watched the telly; people came round for a sing-song," she says she used to "fall out of bed singing."

"Don't try this at home, kids," she laughs, "but I didn't even have to warm up. I would blast through things like Rejoice greatly" - a high, sprightly solo from Handel's Messiah, not your average warm-up of careful long notes and scales. At 17 she was offered a scholarship to the Royal Conservatoire of Scotland; barely three years later, she was spotted by Sutherland and the director Richard Bonynge, and invited to join their Opera Australia ensemble in Sydney.

"I was totally clueless," Buchanan says, shaking her head at the memory. "But the atmosphere in Australia was warm and supportive, like a family." She was graced with the kind of light, instinctive voice that seemed to be able to handle anything: she was a 'happy-go-lucky' performer, she says, and recalls a typical request from Bonynge to learn the role of the Countess in Mozart's The Marriage Of Figaro in a week.

"Really I should have been singing Susannah first," she laughs, referring to the opera's younger, airier female lead. "But back then I just gave everything a go. People were saying, 'Gawd, this girl can sing anything'. Youth is a wonderful thing. I mean, I sang Strauss's Four Last Songs at the age of 19!"

Her effortlessly luminous voice, alluring stage presence and apparent ability to absorb infinite repertoire earned Buchanan a lot of publicity. "You could find out pretty much anything about me without looking too far," she says, still a hint of bitterness in her voice. When she returned to the UK after three years in Australia, she signed with an agency that also promoted the New Zealand soprano Kiri Te Kanawa.

"I think they were modelling me to be another Kiri. Some things about us did overlap, but I didn't have the spinning quality she had - the thing that allowed her to sing Desdemona or Amelia, those wonderful Verdi roles that normally you need a voice of steel to survive.

"I wasn't as comfortable in that repertoire. In Australia I'd been chucked into [Mozart's] Pamina and Fiordiligi... I'd only had three years of training, but I was intrigued by runs and fioritura" - the kind of florid embellishments that, she says, satisfy the same part of her brain that enjoys doing puzzles. "It was a thrill fitting all those little bits together, but I didn't have the instinct for heavier roles. I tried to force my voice to sound heavier when what I needed was to keep the lightness, just keep it going for longer."

She had returned to London with the Australian actor Jonathan Hyde. The pair met in Sydney thanks to former Scottish Opera chief Peter Hemmings, then general manager of Opera Australia, who was presenting a new production of A Midsummer Night's Dream.

"He needed a Puck," Buchanan recalls, "and Jonathan had just been playing Puck for Scottish Opera so they got him over. I'll never forget the first day of rehearsals. He looked so like An Actor!" She remembers the sartorial details vividly: white T-shirt, black jacket, black Panama hat. "And the actorly way he carried himself..." she giggles. "That was 1978. We've been together quite blissfully ever since."

The couple soon started a family in London, and at first Buchanan kept her performance diary full. "You think you're superwoman when you've just had a baby," she says. "I went off to sing Ilia [in Mozart's Idomeneo] for the opening of the new Théâtre Royal de la Monnaie in Brussels when my first child was six weeks old. I remember breastfeeding in the wings at Glyndebourne... It was so stupid. I don't recommend it to anyone."

Increasingly, she found her attention overstretched and her voice struggling in repertoire that had previously felt effortless. She went for tests and was told she had a mildly under-active thyroid; what she didn't realise was the implications for the vocal chords, which can thicken and cause the timbre of the voice to drop. "If I'd known that I would have been banging on the door, making them do something about it," she says. "I was finding it hard to sing high and eventually my voice didn't know if wanted to be up or down. It was extremely painful."

Recounting the experience now Buchanan is frank and candid, but she acknowledges "there were times when I simply couldn't see a way out of it. I thought I must be an idiot. And do you know, not a single industry person knocked on my door to ask whether I was ok? To ask, 'do you need help? What the hell has happened to you, Isobel?' Sure, people are busy with their own careers, but it's interesting that people clamour for you when you're on form then don't want to know when your luck is down."

Buchanan's thyroid condition was properly diagnosed 11 years ago. She was given appropriate medication and her voice eased up, but she has never fully returned to the opera stage. Now her career involves teaching and select recitals: in Orkney, she presents a programme of First World War song and poetry - music by Gurney, Weill, Butterworth and Poulenc, words by Sassoon, Kipling, Owen and others - alongside Hyde. When they premiered the recital at Bath International Music Festival last summer, it was the first time the couple had performed together since A Midsummer Night's Dream back in 1978.

Buchanan is full of enthusiasm for the recital format - "we survived the first performance without filing any divorce papers," she jokes - and the pair are already planning another programme around Shakespeare song settings. "At 61, I am plainly aware of age and stage," she confides. "I'll probably never sing opera again. Never say never, though, and I do wonder about something hearty, something edgy, something that would allow me some real acting possibilities. I thought about [Janacek's] Kostelnicka, for example. I don't think you get to be too cheerful in your sixties..."

She lets out a raucous laugh, shrugging her shoulders, blithely defying her own theory.

Isobel Buchanan and Jonathan Hyde are at Stromness Town Hall on June 20, part of the St Magnus Festival. www.stmagnusfestival.com