The sentence will, to some extent, be determined by the exact charge brought by the Crown Prosecution Service. Looking through the list of possibilities on their website
http://www.cps.gov.uk/legal/section5/chapter_c .html
suggests that the fractured cheekbone might lead to a 'grievous bodily harm charge'. (See Section 5 of that page). If so, the CPS have to decide whether 'with intent' should be added onto the basic GBH charge. (See Section 6 for the criteria which they'll use).
You can read the sentencing guidelines which the judge must consider, here:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
My own guess, based upon reading that document and studying court cases, is that a sentence of around 4 years imprisonment is likely
Chris