Sandy Denny in 1972. Photo: David Bailey/Wikimedia Commons

She died, a burnt-out case, 40 years ago this week. She was 31 years old. The proximate cause of her death was a cerebral haemorrhage as a result of a fall downstairs. But her life had for a long time been a downward spiral of drink, drugs and loss of control.

Professionally it seemed to be all over. Her voice had gone (she had chain smoked for years); her weight had ballooned; her recording contract had not been renewed; her last tour, an attempted comeback, had failed. The money had run out. There was no work in sight.

Her personal life was in even worse shape. Most of her friends had despaired of her and of the mess she had made of her life. Her marriage, shaky at the best of times, had run its course: the day before her death her husband had, without telling her, flown to Australia on a one way ticket, taking with him their nine month old daughter whom she had proved incapable of looking after.

Many of the rock musicians who died young in Britain in the 1970s were the best of their generation. Keith Moon was the most inventive drummer. Jimi Hendrix the most versatile guitarist.

Sandy Denny is, quite simply, the most outstanding female singer that Britain has ever produced.

Born into a middle class family in Wimbledon she began singing in folk clubs in the late 1960s while studying at Kingston College of Art (where she was, without knowing it, a contemporary of Eric Clapton). The recordings which survive of her performances then are mostly of traditional material, which she sang to her own guitar accompaniment.

In 1968 she joined the band with which she is particularly associated, Fairport Convention. They had until then played mainly American contemporary music including Bob Dylan, Joni Mitchell and Leonard Cohen covers.

Sandy Denny transformed it. She still sang Dylan songs with them and was throughout her career one of the finest deliverers of his music. Her interpretation of “I’ll keep it with mine,” for example, which appeared on her first Fairport album What we did on our holidays is vastly more penetrating than Dylan’s own throwaway version. But following her arrival Fairport began playing her, and her fellow band member Richard Thompson’s, own compositions

One of them was the song for which she is mainly remembered. She wrote” Who knows where the time goes?” when she was 19 and it appeared on Fairport’s second album Unhalfbricking before becoming the title track of an album by Judy Collins. The royalties from it was her most constant source of income. It was heard recently on the London stage as the accompanying music to Jez Butterworth’s Jerusalem.

But it was her third Fairport album, Liege and Lief, issued in 1969 which marked the breakthrough. By then she was the dominant figure in the band and its sole vocalist. Liege and Lief marked the beginning in Britain of what came to be called folk-rock: traditional songs performed with an accompaniment of three electric guitars, electric violin and drums. Nothing like it had been heard here before.

But almost as soon as the album was completed Sandy Denny moved on again. She formed a new band “Fotheringay” with her husband Trevor Lucas which only lasted a year; and then became a solo artist, releasing three albums, before briefly rejoining Fairport Convention in 1974. But that too only lasted a year and she returned to solo performing.

Her success during her lifetime was limited. She was twice voted by Melody Maker Britain’s best female artist. She sang with Led Zeppelin, the only outsider ever to do so. But she never achieved the status of Joni Mitchell, the artist to whom perhaps she bears the most resemblance. Even today, when there is more interest in her legacy, she is something of a cult figure.

So why remember her?

Because hers was, in the words of a survey in the Times in 2014, the voice of Albion. A voice which through its purity and its melancholy managed to invest everything she sang, including her own compositions, with the mysteries of the past.

It was, as The Who’s Pete Townshend, a friend and occasional lover, put it “the perfect British folk voice. Not a trace of vibrato. Pure and easy.”

Her sense of melody, her timing, her ability to hold a note, her seemingly effortless breath control were extraordinary. Ralph McTell, who was occasionally one of her backing musicians, called it “heartbreakingly beautiful, cracking in just the right place to touch the emotional heart of the song.”

To listen to her haunting performances of “Farewell, Farewell” on” Liege and Lief or of “Blackwaterside” on her first solo album The North Star Grassman and the Ravens is to hear a raw pain, a desolate sadness of a kind which no other artist in the field of popular music has ever replicated.

She herself insisted that it was not the voice that mattered, it was the song, that the whole point of singing was to tell the story of the song. Many of the songs she sang, including ones she wrote herself, were complex and allegorical. Unlike many singer-songwriters she revealed little of herself in them. It was the voice that gave them their form and their resonance.

Dylan once cryptically claimed that “traditional music is the only true valid death.” Sandy Denny’s actual death was a dismal one. Maybe, if Dylan is right, it was in her music that she truly died.