WHEN York psychotherapist Heather Stephenson-Snell's ex-boyfriend began a new relationship, it sparked an obsession that ended in tragedy, a court heard.

She is alleged to have embarked on a campaign of intimidation, and took up shooting lessons in the months leading up to a fatal doorstep confrontation.

But Stephenson-Snell killed the wrong person when she travelled to Lancashire on Hallowe'en wearing a Scream mask and armed with a sawn-off shotgun, jurors at Manchester Crown Court were told.

Charles Chruszcz, prosecuting, said Robert Wilkie, 43, was "in the wrong place at the wrong time" when he died on November 1 last year in Holland Street, Radcliffe, near Manchester.

He alleged that 45-year-old Stephenson-Snell, of Crombie Avenue, Clifton, had "gone out that night in order to kill another person" - Mr Wilkie's next-door neighbour Diane Lomax.

Stephenson-Snell has pleaded not guilty to murdering Mr Wilkie and attempting to murder Ms Lomax.

Opening the prosecution, Mr Chruszcz said Stephenson-Snell was dressed in dark clothing and wearing a mask from the Scream films when she turned up at Ms Lomax's house at 12.40am.

The court heard Ms Lomax was the girlfriend of Adrian Sinclair, who the prosecution allege had been in a relationship with Stephenson-Snell.

Mr Chruszcz said Mr Sinclair and Stephenson-Snell met in spring 2002 after she advertised for a "dog sitter" to look after her two Rottweilers at her York home.

Mr Sinclair ended the relationship after Stephenson-Snell went on a trip to New York.

Mr Chruszcz said a campaign of intimidation began, including phone calls, once she knew the identity of Mr Sinclair's new girlfriend.

But the campaign stopped in the spring of 2003, said Mr Chruszcz. "What she was doing was taking shooting lessons. She had started shooting in Rufforth."

On November 1, Mr Chruszcz said Mr Wilkie had confronted Stephenson-Snell on Ms Lomax's doorstep about the noise she was making.

He said: "He told her in no uncertain terms that she should get the (Scream) mask off. He grabbed the mask and it came off Stephenson-Snell into the road."

He said Ms Lomax recognised the face of Stephenson-Snell from a photograph.

Giving evidence, Ms Lomax said she looked through a spy hole at her front door to see a person wearing the mask. She said she opened the door as the mask came off.

She said: "The face was fully covered. Robert Wilkie was wearing boxer shorts and nothing else.

"They were arguing a bit. He tried to rip the mask off. When the mask came off, he said to me 'do you know who she is?' I said I know exactly who she is."

Ms Lomax said she was shutting her door when she heard "a bang". When she opened it, Mr Wilkie was "on all fours, gasping for air." He later died.

Mr Chruszcz said Stephenson-Snell was found by police with a bag containing three shotgun cartridges and a light-coloured bloodstained sheet, beneath which was a sawn-off shotgun.

He said officers found a knife in the waistband of her trousers. Stephenson-Snell said they belonged to her boyfriend.

The jury were also told that police later went to a house in George Street, Selby, where it was claimed Stephenson-Snell had left belongings.

In a silver metal briefcase, Mr Chruszcz said two knives were found and written evidence which showed that "what happened in Holland Street was planned and deliberated and calculated by an obsessive person".

Mr Chruszcz said Stephenson-Snell was also the president of a female chapter of the Hell's Angels bikers club.

He said she used the garage of her home as a small nightclub for her biker friends.

The trial continues.

Updated: 10:13 Tuesday, September 07, 2004