You probably know this but GBH with intent is a very serious offence and the maximum penalty is life.
In 2003 the average sentence was 50 months and the minimum was 6 months.
Even with current overcrowding it's pretty inconceivable for him not to go to prison if he's found guilty of this.
I'd say it will most likely depend on who the witness is, how believable they are and exactly what they say.
I think you need the opinion of a good lawyer who's looked at this in detail