Sheila Watson grew up at Balmoral, the daughter of the royal estate’s stonemason, and as a teenager she quickly became aware of Princess Margaret’s romance with Peter Townsend, frequently seeing them together. She was later to reflect: “Little did I realise then the far-reaching repercussions that can result from a man and woman simply being in love.”

Nobody could have spoken from greater experience. She became the wife of a wealthy farmer, Maxwell Garvie, and they had three children. But the marriage became troubled through Garvie’s obsession with sex, pornography and nudism, Sheila complaining to friends that he made abnormal sexual demands on her.

Then she met Brian Tevendale, a 22-year-old barman who became her lover with her husband’s full approval. Garvie would toss a coin to decide which of them would sleep with her at his farmhouse near Fordoun, Kincardineshire, and at his suggestion the trio later had three-in-a-bed sex.

Meanwhile Tevendale’s married sister Trudy Birse had become Garvie’s mistress, Garvie taking her up in his plane and having sex with her while the aircraft was on automatic pilot. He made no secret of this, asking Sheila why she wouldn’t let him perform with her the sex acts Trudy permitted.

Tevendale was infatuated with Sheila and resented having to share her with her husband. Then on MAY 14th, 1968, Maxwell Garvie disappeared, and three days later Sheila reported him missing. She then began to be seen with Tevendale with increasing frequency, and people began to talk.

Meanwhile she had told her mother that Garvie was dead. She was later to claim that she hinted that she herself was involved, thinking this would ensure her mother’s silence. It didn’t. On August 16th her mother had a row with her over her relationship with Tevendale, and then went to the police. The couple were arrested, and the next day Garvie’s body was found in a disused tunnel at St. Cyrus. He had been shot in the head.

Sheila, Tevendale and a friend of Tevendale’s were charged with murder, and at their trial the friend told how he had helped to dispose of the body after Tevendale shot Garvie as he slept. Sheila denied complicity, saying she knew nothing of the murder until after it was committed, and had then kept quiet about it in order to protect Tevendale because she felt morally responsible.

The case against Tevendale’s friend was found “not proven,” but Sheila Garvie and Tevendale were both convicted of murder and sentenced to life imprisonment. After their release 10 years later Sheila continued to protest her innocence, but Tevendale claimed the murder was her idea.