I answered a similar question recently, here:
http://www.theanswerbank.co.uk/Law/Question537 820.html
If you look at that post, you'll see that it was based upon information from the Crown Prosecution website and that of the Sentencing Guidelines Council. I've also done a bit of court reporting for the local press, so this post is (hopefully) slightly better than pure guesswork.
If you've got a reasonably clean record, and a really good barrister, you might be able to keep the sentence down to 18 months (of which you'd serve 9). More realistically, you're probably facing a sentence of around 3 years (with18 months 'inside'). If you've got previous convictions for violence, the worst that could happen might be an IPP sentence. That's similar to a life sentence, where the judge sets a tariff for the minimum term that you'll serve in prison before being considered for release. If an IPP was passed, you'd probably spend around 4 or 5 years in prison.
Chris