A LEADING psychiatrist has called for changes in the Mental Health Act following the horrific killing of an Irvine teenager.

The High Court in Glasgow heard that Miss Angela Thomson, 17, of the town's Mull Place was bludgeoned to death and decapitated in the back garden of boyfriend Brian Fairnie's home - just a month after he discharged himself from a psychiatric ward.

Fairnie, 34, was charged with her murder but, before his trial began, the judge, Lord Caplan, ruled he was insane and unfit to plead or stand trial.

The case went ahead to allow the judge to examine the facts and decide whether Fairnie was guilty or not guilty of murder.

Yesterday, Lord Caplan said that beyond any reasonable doubt Fairnie had killed Miss Thomson but ruled he would have been acquitted of murder because he was insane at the time.

He ordered Fairnie to be detained at the State Hospital, Carstairs, without limit of time.

As the slightly built killer was led from the dock clutching a bible, Miss Thomson's sister, Catherine, 19, who discovered her headless body, screamed at him: ''I hope you roast in hell.''

During three days of evidence, the court was told of Fairnie's violent background and how he believed Miss Thomson was a witch and an alien.

He spoke of having spirits in him, seeing evil shadows on the walls of his home which were trying to kill him, and claimed to friends his teenage girlfriend was dabbling in voodoo and black magic.

A hushed court heard medical evidence of the ferocious attack carried out on the young woman on June 24, during which her skull was crushed with a baseball bat and her head hacked off with a pole from a ''whirlygig'' clothes drier. A policeman told of his search for the severed head, which he found beside a knife under a pile of rubbish bags.

It emerged that Fairnie had spent a short time as a psychiatric patient in Crosshouse Hospital near Kilmarnock - but that doctors had neither reason nor power to detain him.

At the end of May, Fairnie, who had been having intermittent contact with psychiatric services since 1992, walked into Crosshouse Hospital casualty department in a distressed and psychotic state. He was informally admitted but absconded and headed into Glasgow Central Station, where he was found behaving in an odd way.

He was taken to Stewart Street police station where he told a police doctor he was the ''Son of the Father''.

He was returned to Crosshouse to be compulsorily detained for 72 hours for psychiatric examination, during which time he appeared to settle down. Despite pleas from his worried brother for him to be further detained, doctors allowed him to stay on as a voluntary patient. One hour later, Fairnie discharged himself.

Dr Charles Lind, medical director of Ayrshire and Arran Community Health Care NHS Trust, said many aspects of mental health legislation would have to be ''revisited'' and he called for a full debate involving the medical profession and the public.

The consultant psychiatrist also warned that, unless changes in the law were made, similar killings could happen.

He told The Herald: ''I and many of my colleagues are becoming increasingly concerned about the apparent incompatibility of sections of the Mental Health (Scotland) Act 1984 with modern attitudes to mental health.

''I believe we are dealing with many different cases and are living in a society which has a wholly different attitude to mental health than the one which existed almost 15 years ago when this legislation was framed. As well as change in attitude, which I take to be less tolerant, there are also the effects of care in the community to be taken into account.''

Although no exact details have been released, it appears the doctors dealing with Fairnie felt his condition may have been related to drug or alcohol abuse and as such they were not in a position to detain him.

Dr Lind added: ''If we look in general at cases involving alcohol or drug abuse leading to apparent mental instability then we are not permitted to detain any patient unless it is clear that there is illness underlying the effects of the intoxication.

''Even if there is a strong case for detention then the limit is 72 hours.''

He added: ''We also have to take a close look at the difference between dangerous behaviour and mental illness and to be more specific in how each is dealt with.''

His view was backed by the trust, which expressed ''deepest sympathy'' to Miss Thomson's family.

A spokeswoman added: ''Due to patient confidentiality, we cannot give any specific details in regard to this man but we will be looking at every aspect of his treatment whilst he was under our care.

''We believe at the time our doctors acted entirely appropriately. Though in the light of this tragic event, the trust intends to conduct a comprehensive examination of procedures.''

The Thomson family's MP, Mr Brian Donohoe, is calling for the results of any inquiry to be made public in order that public concerns can be allayed.

He said: ''What we must now find out is why the treatment of this man was not completed and why the danger to the public was not identified sooner.''

Miss Thomson's sister, Catherine, said last night: ''He is away to a hospital to get treatment because he is sick in the head. That does not bring my wee sister back. She was innocent and she didn't listen when we warned her off this man.

''I hope he rots in hell.''