this is a hard one then as you don't know what your g/f has said to them,
remembering that she is scared of her ex and what he will do to her if she has said anything about him ,
i am going to give you the worst case scenario,she has told then you hit him with a bottle never mentioned that he pulled a knife on you 1st
the maximum sentence for a section 20 is 5 years
Up to five years imprisonment on conviction by jury trial or six months imprisonment and/or fine if dealt with by magistrates.
It is an offence to unlawfully and maliciously to wound OR inflict Grievous Bodily Harm with or without a weapon on any other person. Instances that amount to a charge of Wounding or inflicting GBH include permanent disability, broken bones, substantial loss of blood. Splitting of skin within the mouth is sufficient. GBH = serious bodily harm, not necessarily a permanent injury.
The section 20 part refers to the relevant section of the Offences Against The Person Act 1861.