A YORK murderer known as the “Halloween Killer” has failed in her bid to spend less time behind bars.

Heather Stevenson-Snell dressed herself in a ghost costume, donned a Scream mask and armed herself with a sawn-off shotgun when she set out on October 31, 2003, to kill her ex-boyfriend’s new partner in the middle of the night.

But instead of killing the woman, she shot the woman’s neighbour who was a “wholly undeserved victim”, Lady Justice Macur told the Court of Appeal.

Psychotherapist Stevenson-Snell was given a life sentence in September 2004 with a tariff of a minimum of 22 years behind bars before she could get parole.

Seventeen years later, she launched a bid to persuade the Court of Appeal to review her sentence on the grounds that her character is now “radically” different as she had made “exceptional progress” while behind bars.

A single appeal judge rejected her bid and when she renewed her application to the Court of Appeal, three appeal judges sitting together on Thursday also rejected it.

Announcing the court’s decision, Lady Justice Macur said defence suggestions that lawyers could collect evidence in the UK and abroad to support her case were “entirely speculative”.

She also said the application was 6,000 days too late.

Appeals against sentence usually have to start within a month of sentence being passed.

Stevenson-Snell, now 65, formerly of Crombie Avenue, Clifton, was convicted by a jury at Manchester Crown Court in 2004 of murdering the neighbour, 43-year-old Robert Wilkie, and attempted murder of the woman.

Manchester Crown Court heard how after a brief sexual relationship with Stevenson-Snell, her ex-boyfriend moved to Manchester where he initially lived with his new partner.

But Stevenson-Snell had threatened and harassed them, particularly the woman, until the ex-boyfriend moved out of his new partner’s house in 2003.

For months, Stevenson-Snell had planned the murder of the woman, taking shooting lessons at Tockwith.

On October 31, disguised in the sheet and the Scream mask, she had travelled to Manchester where she banged so loudly on the woman’s door shortly after midnight, Mr Wilkie had confronted her to remonstrate about the noise.

The woman also came out and when Mr Wilkie pulled off Stevenson-Snell’s mask, the woman recognised Stevenson-Snell and retreated back indoors.

Stevenson-Snell then fired the shotgun into Mr Wilkie’s stomach, killing him.

She left, but police stopped her in her car together with the shotgun, cartridges and bloodstained sheet as she drove back towards York.

The Court of Appeal heard that the judge who sentenced her, Mr Justice Wakerley, had read a psychiatric report on her, which described her as having a “histrionic” personality disorder.

It said she was prone to self dramatisations, exaggerated emotions and was manipulative.

He said she had told “breathtaking lies” in court and shown no remorse.