I think you should consider yourself lucky that you have only been charged with a section 20 and not an 18.
My partner was found guilty of a section 18 and got 6 years. The 'victim' attacked him with a knife and ended up with a fractured skull, after my partner hit him once with a piece of wood.
So my friend again count yourself lucky but i would also say to you that you are definately going to prison probably for around 3 years. Unless of course you can convince a jury that you fractured his skull by pushing him away lol!