In 1982, investigative journalist Marjorie Wallace visited teenage twins on remand for vandalism, arson, and theft.
June and Jennifer Gibbons hadn't spoken to their parents, teachers or fellow pupils for years, as she recalls: "The two girls came in proppped like two stone effigies on the shoulders of the wardens, eyes cast down, lips sealed, with no response."
But at the end of her visit, Wallace mentioned having read their poems and stories - handed over in bin liners by their desperate father. A smile flickered on June's face, she lifted her eyes and asked: 'D..d..d..did you like them?'
The Highgate journalist had achieved the "breakthrough" countless people could not.
"I later read in their diaries that at that moment they decided to trust me. They couldn't talk because of their pact of slience; if one broke it, they would lose the power they had against the outside world. If one dared speak out, the other would gag them. But they wanted to be famous, and trusted me to read what they had written, and tell their stories so the world knew about them."
Wallace continued to visit them in Broadmoor psychiatric hospital where they were detained indefinitely. She waged a "long, frustrating campaign" in the Sunday Times against the "shocking injustice of their sentence", and covered Jennifer's inquest when she died in mysterious circumstances upon their release.
The "strange enigma" of their intense relationship inspired her 1986 book 'The Silent Twins' which spawned an opera, a TV drama, a play, and now a film starring Letitia Wright and Tamara Lawrance.
Born in 1963 to Caribbean parents, the identical twins grew up in Haverfordwest and from an early age devised their own private language, refusing to read, write or communicate with others.
They were sent to separate boarding schools, but became catatonic. Reunited, they spent hours in their bedroom making up stories with rag dolls and writing diaries, poems, and novels. Wallace describes it as a "crucible of self-invention and creativity."
"They had left school uneducated but were reading the classics, they bought a typewriter, took a correspondence course in writing, and were launching themselves secretly on their literary career."
When American boys from the nearby naval base introduced them to vodka, glue-sniffing, arson and sex - then abandoned them - they went on a five week vandalism spree setting fire to empty buildings.
"It was pathetic what they stole; half a packet of polo mints; it was a cry for help. They badly needed psychiatric care, but no-one would take them except Broadmoor. They were convinced it would be their salvation, their cry would be answered, and people would understand them. But it wasn't like that."
The twins smuggled their diaries out of Broadmoor and Wallace painstakingly deciphered the tiny writing, revealing the sisters' "intense rivalry and extremes of love and hate for each other."
"They were a danger to each other, it was going to end in tragedy, they couldn't be separated and couldn't be together."
Chillingly the girls had decided the only way out was for one to die.
"We were having tea in Broadmoor and suddenly Jennifer said 'I am going to die.' June was nodding, and when Jennifer gave me a poem she was smiling. I had never had a more chilly feeling. I knew something would happen, they had decided one had to sacrifice their life to liberate the other."
Wallace says Jennifer's death aged 29 is an unsolved mystery. She remains in touch with June who is convinced it was their decision.
"She says 'I need to go on and live for her'. She's the most remarkable person, very calm and poised with a wry sense of humour. You would never know that she had spent 11 years in Broadmoor. When I asked why hold onto their silence for so long she said: 'I really don't know what made us do it, we were lost girls who were trapped with each other, no-one could break away without it leaving the other completely desolate.'
"She's at peace now and doesn't want to be disturbed."
Wallace, who is founder and chief executive of Islington-based mental health charity SANE, believes if she hadn't pursued the twins' story it would not be known.
"No-one had taken the time or patience to read their work and understand what lay beneath that wall of silence. It's important that a strange story like that is told, that people who behave in bizarre ways are not judged and written off without exploring further. What might we discover if we listen and excavate the complexity and richness of people's imagination?"
An executive producer on Agnieszka Smoczynska's "powerful and moving" film, she praises Wright and Lawrence's acting as "superb."
"The director has made a magical fantasy story in the twins' minds. I felt I was portrayed as more passive, not as a tough campaigning journalist, but I am not the most important part of the story. As long as it tells the truth and does justice to the girls story. I accept it for its own worth."
The Silent Twins is in cinemas from December 9 and Marjorie Wallace's book is reissued by Penguin.
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