This, it turns out, is no ordinary copy: it is the one Ted Hughes gave to Elizabeth just a week after Plath killed herself by putting her head in a gas oven. Because Elizabeth, the person to whom she had dedicated the novel, was Plath's best friend. She was her wise "earth mother", the woman Plath fled to in rage and despair when she discovered that her handsome husband, the father of her two children, Nick and Frieda, was having an affair with another poet, Assia Wevill. "My milk has dried up," she sobbed. "Ted lies to me all the time. He has become a little man." After Plath's death, it was Elizabeth who moved into the couple's marital home in North Tawton, Devon, at Hughes's request. And it was Elizabeth to whom Wevill sent the gas bill for the period covering Plath's suicide at her London maisonette. (She was your friend, was the chilling implication: you pay the bill.)

Most people know the bones of the Plath-Hughes tale but few have as vivid memories of it as Elizabeth. "I felt as if Sylvia was my utterly brilliant younger sister," she says affectionately, in her musical, old-fashioned voice. "She was a person of importance: you could feel it and see it when you were with her. You knew she was special. "I think Gwyneth in the film is very like Sylvia. She's got that inwardness, a privacy about her, that Sylvia had. But, of course, Daniel Craig couldn't be Ted. There wasn't anything in the script that allowed him to be the charismatic person that he was. Ted was very big, he was very dark, he had this big Yorkshire voice and this Heathcliff quality. I don't think you'd have met many women who weren't starry-eyed and gaping at him. Women thought he was the most devastatingly attractive man they had met."

It is obvious what Plath saw in Elizabeth. She has a sharp intelligence and honesty, and evidently loves life. She once ran a shelter for heroin addicts and has campaigned against the use of chemicals in agriculture. The film company Miramax employed her as a consultant during the making of Sylvia. (which has just opened), But, though she befriended Paltrow, she was disappointed by the film's portrayal of Ted's and Sylvia's relationship. "It doesn't show their happiness," she says. "Their house, for instance, wasn't some dreary cold comfort farm. Court Green was a lovely old place in the village, with a beautiful garden and orchard. And Sylvia was so alive." Four decades after she died, the Plath controversy still rages, its central preoccupation unchanged: was Plath sinned against, or sinning? Did her demands and jealousy drive Hughes away, or was she the victim of his wandering eye?

The two views seem irreconcilable. Frieda Hughes, who champions her father, refused to co-operate with the making of Sylvia or allow Plath's poetry to be used. On the eve of the film's release, she attacked the way Hughes had been "vilified by feminists who want to blame him for the suicide of my mother, and so adopt my mother for their own devices". She drew a parallel between his hounding and the accusations levelled at Prince Charles over the death of Diana, Princess of Wales. Elizabeth's disapproval is clear but she chooses her words carefully. "I think Frieda's got a very hard path to travel," she says. "But the Charles thing is nonsense."

Elizabeth first met Plath and Hughes in 1962 after hearing them lament on the radio that they had no room to work at their tiny London flat. Her offer of accommodation was turned down - instead the couple bought Court Green, half an hour from Elizabeth's house - but they became friends. Elizabeth talks of Plath's radiance in the early days in Devon. She would enthuse about the children she and Hughes were going to have. So why was he unfaithful? Elizabeth sighs. "I think he was flattered because Assia was stunningly beautiful. He probably hoped he could have a quick fling and Sylvia would never find out. And, of course, he was insecure." Plath wanted to get back with Hughes after their separation, Elizabeth says. In the film he tells her they can't because the 35-year-old Assia is pregnant, though Elizabeth is not sure if Plath, in fact, knew about that.

"And if anyone told her," she says, "it would have been Assia, knowing what she was like. I've never come across anyone like her. She had no moral sense at all. She was like a very beautiful wild animal." Within a week of Plath's suicide, Hughes and Assia had moved into Plath's flat in Primrose Hill and were even eating in the kitchen where she had gassed herself. Why did they do that? Elizabeth shakes her head. "I thought it was dreadful. I couldn't understand. I could see Ted didn't have the strength any more to fight Assia - I don't think he knew what he was doing. He had to look after the children, and all their stuff was there. And I think once Sylvia died, Assia decided she was going to become Sylvia." A week later, though? She nods. "It was terrible. I went there to see the children. A very nice girl was looking after them. She said, 'You know Mrs Wevill's living here?' I said, 'In this flat?' She said, 'Yes, and she's having an operation - one of those.' I said, 'You mean an abortion?' She said, 'Yes, Mr Hughes has taken her to hospital.' They came back and Assia went up to Sylvia's bed. Ted came in and handed me a copy of The Bell Jar, because it was dedicated to me and my husband. He boiled a kettle for a hot-water bottle for Assia, stood back against the window and said, 'It doesn't fall to many men to murder a genius.' I said, 'But you haven't.' He said, 'Just as well,' meaning, 'I might just as well have.' "

Shortly afterwards, Elizabeth and her husband moved into Court Green to take care of the house for Hughes. He rang one day and asked if he could collect some things. "When he arrived, of course, Assia was with him. She didn't want Ted to come and be there on his own. She said, 'Ted, will you ask her to show me round?' Ted looked down at his plate and said, 'Would you mind, Liz?' " Elizabeth stiffens with outrage and her voice rises. "I had to - it was his house. We went up the stairs and stopped outside Sylvia's workroom, which was padlocked. She said, 'Don't you feel a traitor?' Laughing, smiling. I said, 'Yes, I do, and I'm not going any further.' I went downstairs. Ted was in tears. Assia looked at him and looked at me and said, 'Elizabeth, do you think Ted and I can be happy?' I said, 'Just look at him. Sylvia will always be there between you.' "

What about the gas bill Assia sent her? Elizabeth shakes her head with horror. "We didn't have a photocopier, otherwise I'd have photocopied it. But she really did do that. It was the North Thames Gas Board, and a little note at the bottom said, 'I believe this covers your occupation of Court Green.' I opened it and saw that this was the period when Sylvia had died! "When Ted rang up, I told him. 'Oh, stupid girl,' he said, 'she does get in a muddle.' " So it was a mistake? Elizabeth shakes her head grimly. "No, it was because Assia knew I was Sylvia's friend, and I loved her. You see, this came after the scene where she'd asked me if she could be happy with Ted. She wanted to hurt me."

She sniffs - even after four decades, talking about how Plath affects her. "Of course, Ted had demons," she points out. "He was into black magic. He said to me at a party: 'My creativity presented me with a demon. If I get close to people, I destroy them.' I think he was tormented." Elizabeth's prediction about the fate of Wevill's relationship with Hughes came true. They never married and, in 1969, Wevill, too, gassed herself, taking her two-year-old daughter, Shura, with her. "She was getting fat and going grey by then," Elizabeth says. "She complained that Ted's friends couldn't accept her because they felt she was the cause of Sylvia's death."

Does Elizabeth think the tragedy should be laid to rest? "The academic research will go on because Sylvia Plath was a very important poet. But I think it would be far better if the personal details of Ted's and Sylvia's life were let be, because what good can it do, bringing it up again?" - The Sunday Telegraph