Section 18 offences carry a maximum penalty of life imprisonment.
The sentences that judges have to pass (other than in genuinely exceptional circumstances) are given in this document:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
The sentences shown in the table on page 13 apply to first-time offenders, facing a single charge, convicted after a trial. An early guilty plea will see the sentence pulled downwards, as will the element of provocation. However, if you're facing more than one charge (which isn't clear from your post), that will push the sentence upwards.
Without hearing the prosecution's side of things it's hard to be precise but I'd hazard a guess at a sentence in between 2� and 3� years imprisonment.
Chris