It took Marian Partington 10 years to write a letter to Rosemary West. It was sent 31 years after the murder of her sister Lucy, whose dismembered body had been found beneath concrete in the basement of 25 Cromwell Street, Gloucester.

“When my sister Lucy disappeared from the bus stop near Pitville Park in Cheltenham she was 21 years old and you were 19 years old”, the letter began. “I don’t know the whole truth about Lucy’s murder. You have denied being involved in it.

“I have not met you, and know very little about you, apart from the crimes which you deny committing and the abuse that you suffered as a child.”

The lengthy letter expressed the hope that the wife of Fred West was being “helped and supported” and explained how Ms Partington was seeking “to know how to be free of any feelings of anger and bitterness towards you”.

The reply from the prison on behalf of Rose West was altogether briefer.

It simply said: “Ms West has received your letter and asked me to relay a message on her behalf and asked that you please cease all correspondence. She does not wish to receive any further letters from you. Any further letters will be kept in security.”

Ms Partington, whose family moved to Oxford where her father studied chemistry at St John’s College for a doctorate, has spent much of her life trying to come to terms with first the unexplained disappearance of her younger sister, then the discovery of Lucy’s body and ultimately the terrible void left in her life.

Now she has written a book, If You Sit Very Still (Vala £15.99), which explores that traumatic loss and Ms Partington’s own journey, which has culminated in 10 years of restorative justice work in prisons. But the book is also an attempt to reclaim Lucy Partington, who fell into the Wests’ hands on December 27, 1973, when she was in her final year at Exeter University, from the status of ‘West victim’.

She addresses head on the cruel question that has so often been put to her — whether she wishes her sister’s body had never been found.

“To say ‘my sister was murdered, she was one of the Wests’ victims’ makes my throat ache,” she says. “It was easier to say ‘my sister disappeared’, but more difficult to live with that sense of unresolved loss. However, the grotesque details surrounding Lucy’s life are part of my life. It is not possible to pretend it didn’t happen. It is hardly possible to understand. But there is something about trying to get the measure of it before one can let go of it.”

Ms Partington, who was born in Oxford 48 years ago, endured 20 years of not knowing whether her sister was alive or dead.

Within the family, who later moved to the village of Gretton in Gloucestershire, the subject of Lucy’s disappearance became almost taboo.

“It was impossible for me to imagine that she had been murdered, let alone talk about it. It was almost as if naming that possibility would be treading on a land mine,” Ms Partington recalls. “We had better not dare to rehearse that scene, because even putting it into words would be tempting fate. The longer we didn’t speak about what might have happened, the worse it felt.”

But all was to change in March 1994, when she received a phone call from her mother, who was to die in a nursing home near Oxford in 2009, warning that newspapers were suggesting that a third unidentified body dug up in the garden of 25 Cromwell Street might be Lucy. Five days later came news from the police that they had “news”. The family were soon to learn that Lucy had been gagged, beheaded and her dismembered body stuffed into a hole surrounded by sewage pipes.

Ms Partington neither spares herself or her readers: “The rope that held her in bondage, two hair grips, a few strands of hair and the masking tape gag survived with most of her bones,” she writes.

Mr Partington was to learn of Fred West’s suicide as she returned from a New Year’s Eve party with her neighbours.

“I never felt cheated by his suicide, although many people thought I should,” she said.

The family — her parents divorced when Ms Partington was 12 — bravely attended the trial, finding it impossible to match the figure of Rosemary West in the dock with the graphic details of sexual depravity and brutality.

The playing of tapes of Fred West being interviewed added to their pain, with the killer claiming Lucy to have been “just a girl I was knocking off”. The thought that even one person might have believed his words was unbearable.

It was during the trial that Ms Partington began setting down her thoughts. In addition to her desire to ‘reclaim’ her sister, her book offers insight into her efforts to move towards acceptance, and even understanding, of what happened.

“I have always been interested in the process of healing through my work as a homeopath. It has helped me move from being isolated and alone. It would not have been healthy to demonise and write off the people who killed Lucy. The Wests were human beings.”

Everything they did to her sister, she believes, tells of something that had been done to them in their childhoods. She admits to being unsettled by the ITV drama about the recent Fred and Rose West murder investigation, Appropriate Adult, starring Dominic West.

“I watched the first part but not the second. I understand that Dominic West quoted me saying that ‘it is important to face this darkness within’, as the reason for doing it. People say the acting was amazing but I just couldn’t understand the purpose of it.”

Ms Partington lives in Montgomeryshire, where she and her husband brought up their four children. She now believes everything that has happened offered an opportunity for change within herself.

“It has been important to try to stay true to my own life.”

An invitation to spend a day at Grendon Prison, one of the UK’s therapeutic prisons, led to her taking a keen interest in restorative justice.

She has been visiting prisons on a regular basis for more than a decade, since 2007 as worker with the Forgiveness Project, sharing her story with inmates and listening to their accounts of their crimes, including murder.

At some point she always produces a little woollen hand-spun bag made of stray sheep wool. This, her most precious possession, has been passed through the hands of many complete strangers. The wool was woven by Lucy when she was eight. Once containing embroidery silk, it now holds prayer beads.

It was made painstakingly and with patience, she recalls, and with a thread that has taken Lucy’s sister half a lifetime to follow.