A few days later, bowed by the strain of both the attack and having to attend a police identity parade, she was taken into a local hospital for psychiatric care. The next thing her mother heard was a call asking her to go to the hospital as a matter of urgency.
'A male nurse in charge took me into a room,' she says. 'He said: 'I'll have to come straight out with it. Your daughter's been sexually assaulted.' I said: 'I know that.' He said: 'No, I mean it's happened again since she has been in hospital.' '
Diane had, almost unbelievably, been attacked again, this time by a male patient on her ward who had voluntarily admitted himself because of his past record of sexual offences against women.
Even so Diane's mother, who does not wish to be identified, here or on the BBC Public Eye film 'Sex on the Ward', in which she can be heard tonight, is still angry about the way her daughter was exposed to danger in a mixed- sex psychiatric ward at her lowest, most vulnerable point.
'I think it's all wrong,' she says. 'Women in those wards have often had problems like my daughter, or with their husbands. The last thing they want is to be on a ward with men.'
The trouble is that mainstream psychiatric practice disagrees with Diane's mother. Since the Seventies it has been considered therapeutic to mix mentally ill women and men together - more natural, more like real life. But in the wake of recent allegations of indecent assaults and even rapes taking place in these supposedly therapeutic environments, the charity Mind has begun to campaign for a reappraisal of the whole policy.
'We have undertaken extensive consultation with women who have experience of psychiatric services,' says Liz Sayce, Mind's policy director. 'Women told us they did not feel safe in psychiatric hospitals, and one of the reasons is mixed wards. There have been a number of alleged rapes taking place in them, and we have many reports of harassment.
'We want to see women given the choice of a single-sex ward. They should be able to feel secure at times of extreme crisis in their lives.'
Nor is it only a question of women needing protection, Mind argues. Men whose mental illness leads them to sexual aggression also need to be protected from committing criminal acts.
Not all men who are mentally ill, of course, turn into rapists. And women patients can be violent, too. But studies have shown that the backgrounds of female psychiatric patients may make them particularly vulnerable to a fear of male harassment.
When the idea of mixing sexes in psychiatric wards became fashionable the study of sexual abuse was in its infancy. Since then repeated studies have pointed to a link between a background of sexual abuse and mental ill health in women. At least 52 per cent of women in psychiatric care, according to some, have been physically or sexually abused in childhood.
For these women admission to wards where men may be merely shouting and gesticulating violently could be traumatic, let alone the effect of actual attack.
How common the risk of sexual violence is on mixed wards is difficult to judge. Few cases come to court. The evidence of female or male victims who are mentally ill is easily called into question.
One mother told me how, after her daughter alleged rape by a visitor to a London psychiatric ward, police interviewed a man and assured her that they were certain the crime had been committed.
But since the girl's health was too fragile to stand the strain of the witness box - and she would easily, as a diagnosed schizophrenic, be made to look unreliable in cross-examination - there was nothing they could do.
Some senior psychiatrists still have a firm belief in the usefulness of mixing sexes. Professor Bruce Pitt, of the Royal College of Psychiatrists, appears in Friday's Public Eye to say that there is such a small incidence of abuse that the problem is insignificant.
Others are not so sanguine. Malcolm Alexander, chief officer of Camberwell Community Health Council in south London, says: 'Over the past five years we have had five allegations of rape in psychiatric wards. Over the past six months we have had two serious complaints from women at the Maudsley Hospital who allege they have been sexually assaulted.'
The Maudsley Hospital, which is investigating the allegations, runs a training course for staff to help them to deal with sexual aggression among patients. But Mr Alexander believes there is only one real answer: 'Every psychiatric hospital should have areas designated for women only. These patients come into hospital severely traumatised. They must be able to feel safe.'
Janet Richardson, chief officer at Hackney Community Health Council in north London, says that three allegations of rape in psychiatric wards have been made to her. None has resulted in charges, but all make her anxious to see women-only areas built in the new psychiatric unit at Homerton Hospital.
Parkside Community Health Council in west London has also expressed concern to its local health authority about how allegations of sexual assault are treated.
Just as fears about the mixed- ward policy are being voiced, it seems, surprisingly, to be about to be extended into a still more potentially dangerous area - Britain's special hospitals, where some of the most dangerous sex attackers are housed.
Ten years ago women at Broadmoor, many of whom have a history of sexual abuse, were pressurised, in the name of therapy, to attend discos and dance with, among others, serial sex offenders - hardly, on the face of it, an activity likely to make the opposite sex appear in a rosier light.
The Broadmoor disco is a thing of the past. But last July the Special Hospital Services Authority reported that one of its objectives is to introduce mixed wards.
Dr Jennie Williams, senior lecturer in mental health at the University of Kent with long experience of work in psychiatric hospitals, has no doubt about the effect of putting mentally ill men and women to sleep side by side.
'Many women don't feel safe,' she says. 'I think the practice has no therapeutic value for women at all. What is happening is a cruel parody of what psychiatric care should be.'
'Sex on the Ward' will be shown on BBC 2 tonight at 8pm.Reuse content