Initially, they were a two-piece, consisting of Fraser and her mercurial guitarist boyfriend, Robin Guthrie. She had never sung in front of anyone before, when, reluctantly persuaded to have a go, she found this ethereal noise floating from her lips, pure as ringing crystal. Around it, Guthrie constructed an unearthly swell of chiming, shimmering, ricocheting guitars that, unlikely as it might sound, provided the perfect foil for Fraser's incomparable voice. Their sound was their own, unique to them.

The pair's first few albums, Garlands, Head Over Heels and Treasure, which appeared at yearly intervals from 1982, contained some sublime music, although Guthrie's tendency to mask the melodies with endless, often indiscriminate overdub squalls could make listening to a whole album exhausting. It was only when recording a version of someone else's song, Tim Buckley's Song to the Siren, under the name of This Mortal Coil, that Guthrie could bring himself to strip away the layers, with extraordinary results. Their haunting reading of this majestic song is unarguably great.

If the music could be indistinct and frustratingly dense, the words Fraser sang were even more so. Her lyrics and song titles (for example, Ella Megalast Burls Forever) had seldom made any kind of literal sense - although, as a friend once noted, you always knew what she meant, anyway - but on Blue Bell Knoll, she went so far as to eschew recognised language entirely, inventing instead her own private lexicon of trills and sighs. Although Blue Bell Knoll was intriguing, it was sailing close to solipsistic self-parody and has aged nothing like as well as its 1990 successor, Heaven or Las Vegas, which is more generous and contains some heart-stoppingly lovely songs.

Part of the Cocteau Twins' appeal was the confused, equivocal nature of their project, which made them seem like a last outpost of sanity in a decade defined by self-certainty. Their music seemed to represent some sort of emotional catharsis, but was at the same time tied to a profound fear of interpretation. This was obviously contradictory, and suggested some dark forces at work behind the sounds, something that turned out to be quite true. Fraser would later speak of having spent the five years leading up to Heaven or Las Vegas "in a coma", mostly induced by the usual rock'n'roll indulgences. In September 1989, as Heaven or Las Vegas was being recorded, the arrival of a baby, Lucy Belle, to her and Guthrie, snapped her out of it. "You see, I always thought that I was having a great time," she said. "I really wanted Lucy, really badly. A lot of women reach a stage when their bodies are telling them it's time to have a baby and I thought that was what was happening to me. In the end, though, I realised that I really wanted a baby because I hated my life."

The change didn't affect Guthrie at that point, however. The chaotic Heaven or Las Vegas tour found him in truculent, unpredictable mood. Thanks to what he later realised were serious addictions, he became nigh-impossible to live and work with. At some point between then and the next album, Four Calendar Café, he and Fraser parted as lovers and she began to build a new life. This had the effect that on Four Calendar Café, you could understand the words and even guess at explicit meaning. What seemed to be there was a great deal of defiance and anger, which some reviewers took to be directed at Guthrie, but was nevertheless parlayed into a strangely beguiling, bittersweet record.

What was clear was that, by the time Four Calendar Café was released, Fraser was again protective of Guthrie, who had finally undergone treatment for serious addictions and reverted to the affable, generous man he had always been underneath. This, he explained, accounted for the unprecedentedly svelte production on that album.

"I've consciously been stripping things back. In the past, I've always wanted one more overdub, one more melody, because I'm terrible for thinking that my music isn't good enough, so if I put in a few more frilly overdubs, then it'll be all right. If you take something like Blue Bell Knoll and strip away the overdubs, you'll find that there's not much there, the songs are made up of lots of little bits of nothing. I've also only felt as good a person as the last record was good. If people didn't like the record, I always felt that it was because they didn't like me."

This must have been an onerous burden for anyone to carry, making the desire to escape understandable. The key to the Cocteau Twins had, indeed, turned out to be the contradiction between a need to be heard and a mordant fear of being understood.

The frightening question is this: was it the final liberation from that painful but productive state that caused the Cocteau Twins' audience to lose interest in them? Or had their natural sell-by date merely arrived? Four Calendar Café was a good record that got a lukewarm reception, while hardly anyone noticed its follow-up, Milk and Kisses, and time was duly called. Their best work had been done during a time of unremitting misery, which makes it heartening to note that, in Manchester, Liz Fraser looked more relaxed than she ever has in public, performing a stirring Teardrop and an encore of Group Four. When not singing, she danced by the mixing desk at the side of the stage, perhaps also buoyed by the fact that, after a scramble for her signature, one of our most talented and likable singers has just signed a solo record deal. Good news for us all.