The woman revealed how she also felt the incident would have been worse if her daughter had been alone, when a reconstruction of the assault was broadcast on BBC's Crimewatch last night.

The 13-year-old girl was approached by a man who seriously sexually assaulted her in John Lea Park, Doddington Road, Wellingborough, at around 6pm on Thursday, August 28.

Fighting back tears in the interview for the programme, the mother said: "She has nightmares. She keeps asking me 'has he been caught, why did he do it to me?'"

She added: "She said she was going out to the park. She'd been gone for about 15 to 20 minutes when she came back to the house and said, 'mum, something's happened at the park'.

The reconstruction then showed how the man told the girl and a friend who was with her that he was a housing officer who was there about graffiti and to come down from the slide and talk to him. He then told the girl's friend to leave before sexually assaulting the teenager.

Her mother added: "My daughter has a disability and I believe he targeted her because of it. He's ruined her childhood. If she hadn't had a friend with her I think he would have done something worse."

An expert into these types of crimes, Prof Laurence Alison, told Crimewatch presenter Kirsty Young the man had had an unusual approach because of his authoratitive tone and that it was possible he has offended before.

Northamptonshire Police have also released an e-fit picture of the suspect.
He is a white man, with short, spiky black hair and a greying goatee beard. He had yellowing teeth, was of slim build and is described as being short for a man. He wore a long-sleeved, light blue top and black trousers.

The man was carrying a black folder and was aged between his mid-20s and mid-40s. Witnesses can contact police on 08453 700700 or Crimestoppers on 0800 555 111.