A FORMER soldier who killed a hillwalker in one of the most high-profile Scottish murders of recent times must serve at least 15 years for his crime, the country's senior judge ruled yesterday.

Donald McMillan, who had a perverted fascination for older women, suffocated Helen Torbet, 62, a gynaecologist's wife, from Busby, Renfrewshire, and buried her body in a shallow grave in the garden of his parents' guesthouse in Wester Ross.

McMillan, who was 31 at the time of the killing, concocted a story about Mrs Torbet driving away from the guesthouse with another man in July 1993, and it was more than 10 months before the body was discovered.

Police at first treated the case as a missing person inquiry until some of the victim's belongings were found only 200 yards from the guesthouse and suspicion focused on McMillan.

McMillan, now 41, was jailed for life at the High Court in Inverness in September 1994 after a jury found him guilty of murder by a majority verdict.

During the trial, he admitted killing Mrs Torbet, but claimed he was only trying to calm her down following an argument and that he should be found guilty only of culpable homicide. He bound and gagged his victim and cut open her nightdress with intent to rape her.

Lord Cowie jailed him for life and also sentenced him to five years for attempting to defeat the ends of justice by inventing the story about Mrs Torbet leaving the guesthouse.

McMillan appeared yester-day at the High Court in Edinburgh so that the court could fix the ''punishment part'' of his sentence under recent human rights legislation.

Lord Cullen told McMillan: ''It was a sexually motivated assault and you must have subjected the victim to a terrifying ordeal. I consider that in your case the appropriate punishment period should be 15 years.''