The document the judge must refer to is here:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
See the table on page 15, but noting that those sentences refer to a first time offender convicted after a trial. (An early guilty plea could see the sentence cut by up to one third).
Based upon that information (and assuming a guilty plea), my guess is a sentence of between 6 and 12 months imprisonment (with only half of that time actually being spent 'inside').
Chris