Simon Richard Bews, 43, a married father-of-three, followed the 17-year-old, then grabbed her bottom and put his hand over her mouth in November 2007.

She was on the phone to her boyfriend at the time, after realising she was being followed. The pair had been walking home together but he moved on ahead after a row.

Bews dragged the girl into the middle of the road but ran off as her boyfriend approached.

She was so scared she thought she was going to be raped, London's Court of Appeal heard today (September 18).

Bews, of London Road, Sleaford, admitted sexual assault and was sentenced at Lincoln Crown Court in May to three and a half years in jail.

However, that was slashed to 18 months today after his lawyer, Michael Cranmer-Brown, argued the sentence was too harsh.

The barrister said that the Crown Court judge had imposed the term on the basis of speculation as to what might have happened if the girl's boyfriend had not turned up.

Mr Justice Nelson, sitting at the Appeal Court with Lord Justice Hooper and Mr Justice Maddison, said Bews admitted he was drunk that night and had been under some financial strain, because business was not good.

He also pointed to character references submitted to the court and described Bews as a 'good family man'.

Mr Justice Nelson said: "We accept it was certainly not surprising that the victim felt petrified that she was going to be raped."

But he added: "It appears he was sentenced on the basis that a more serious sexual offence would have occurred.

"The inevitable anxiety created by an offence like this must be taken into account but a court cannot sentence on the basis that such an event would have taken place, as it is not known.

"The court cannot know if the victim would have been raped, or if the offender would suddenly have desisted. There must be more certainty in sentencing than taking such an approach."