Two stitches in a nose shouldn't merit a GBH charge. See here for the type of injuries that constitute 'GBH':
http://www.cps.gov.uk/legal/l_to_o/offences_against_the_person/#a15
So, if your account of the injuries is correct, the court will throw out the GBH charge, and substitute ABH anyway.
However I'll answer your actual question. (i.e. "what is the likely penalty for a 'Section 18' conviction?"):
The circumstances you describe suggest that the court would regard it as a 'Category 3' offence, meaning that the 'starting point', which a judge has to work from, is 4 years imprisonment. Unless there are really exceptional circumstances the minimum sentence can't be below 3 years and the maximum can't be above 5 years.
The sentences I've quoted refer to a first-time offender who is convicted after a trial. An early guilty plea can see one third deducted from the sentence, while previous convictions (particularly for violence) can see the sentencing range pushed higher. Normally half of the nominal sentence is actually spent 'inside'.
See pages 3 to 6 here for further information:
http://sentencingcouncil.judiciary.gov.uk/docs/Assault_definitive_guideline_-_Crown_Court.pdf
(That's the actual document which judges
must refer to when considering sentence. Scroll down to pages 11 to 14 to see the sentencing guidelines for ABH).
Chris