But questions have arisen over the verdict because Sane, the mental health charity which represented her family, says the jury was denied expert evidence relating to the condition which caused her death - and was given insufficient evidence about her physical care in the days before.
Yesterday, Majorie Wallace, the charity's chief executive, said she was 'very disappointed. Jennifer's death remains as big a mystery as ever'.
The jury at Bridgend coroner's court, Mid Glamorgan, took minutes to return its verdict after hearing that Jennifer collapsed and later died on 9 March, the day she fulfilled her wish to leave Broadmoor - the twins' home for the past 11 years. The sisters had been sent there after admitting conducting a six-week campaign of theft, arson and wanton vandalism in Haverfordwest, Dyfed, in 1981.
Dr Susan Claydon, a pathologist, said Jennifer had died from massive acute myocarditis - inflammation of the heart, seen sometimes in young athletes who have died suddenly. But Ms Wallace said heart experts would have told the jury that the condition has many causes, some viral, some drug related, and that most people who suffer from it are cured.
Philip Walters, the coroner, had declined a request from the family to call more witnesses from Broadmoor who had care of Jennifer. Ms Wallace said this restricted the jury.
The hearing was told that Jennifer had been sick, complained of headaches, and was very tired before starting the journey to a special secure unit in Bridgend. But Dr Lyal Hamilton, consultant psychiatrist at Broadmoor at the time, said she had been examined the day before and was considered fit to travel.
Her sister, June, her vow of silence behind her, told the inquest how her twin had said the day before the journey that she was dying. She thought it was a joke, but as they began their journey Jennifer slumped on her shoulder. She then lay across her lap, 'sleeping with her eyes open'.
A touch of nervousness and a slight speech impediment were the only hint of June's previous life with Jennifer - which inspired a French opera, a film and a book.
The court was told that on arrival at the Carswell special unit, Jennifer's drowsy condition caused concern. Doctors examined her and when blood samples later showed a dramatic fall in red cells she was transferred to the general hospital, where she died at 6pm.
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