A sex stalker who terrorised his victims in a hideous Halloween mask has been given a seven-year extended prison sentence.

Deviant thrill-seeker Gary Mawson targeted young girls on the railway then struck under cloak of darkness, a Court heard.

He will spend four years behind bars and three on extended licence.

The former fitness instructor took cannabis and anabolic steroids before boarding the Wharfedale line to seek out and follow his prey.

Mawson, 31, of Woolcomb Court, Heaton, Bradford, pleaded guilty to sexually assaulting a 14-year-old girl and indecently exposing himself to three teenagers and a young woman between November 7 last year and January 26.

Arrested on February 18, he told police they had caught him “at the right time” because he was a danger to young girls, Bradford Crown Court heard yesterday. He said he bought the red devil mask, found by police at his home, at Halloween.

Mawson told detectives he was overtaken by the desire to commit the offences and felt removed from himself. He stalked his victims on the Wharfedale line to avoid recognition in his home area.

Prosecutor Neil Clark told the court Mawson first struck at 9.30pm on November 7 when he followed two 15-year-old girls from Burley-in-Wharfedale Railway Station.

They heard a noise in bushes and saw masked Mawson performing a sex act on himself.

On December 23, a 22-year-old woman at home alone in her ground floor flat heard someone trying her locked back door.

She saw a hooded figure outside her window wearing a mask reminiscent of Edvard Munch’s The Scream painting. He was committing a sex act on himself.

When challenged, Mawson asked: “Do you want some?”

Five days later, Mawson, wearing a scarf round his face, followed a 14-year-old girl home from Burley station. He called over to her and stood under a street light exposing himself.

On January 9, Mawson, with his lower face covered, stalked two girls aged 14 and 15 from the station at 6pm. He pushed the older girl into the road and sexually assaulted her friend, fleeing when she screamed.

On January 26, a girl of 13 saw Mawson in the waiting room at Menston Station and recognised him as a man who had tried to take photographs of her on the train.

Mr Clark said the train guard recalled seeing Mawson acting strangely and sitting near to young girls.

In 2002, Mawson was sentenced to a community order for making obscene phone calls to three women clients at a Keighley gym where he worked as a fitness instructor.