Start by taking a look at the sentencing table on page 17 of this document:
http://www.sentencing...person_accessible.pdf
You'll see that the 'starting point' sentence (which a judge has to work up or down from), where there was no pre-meditation and non-permanent injuries, is a non-custodial one of a 'high-level' community order. However the sentences shown there refer to a first-time offender who has been convicted after a trial. Previous offences, particularly for violence, will push the sentencing upwards. Courts also take a dim view of offences of violence against victims who are deemed to be 'vulnerable', which can include people who are abused by their partners.
So I'm guessing at a prison sentence of between 3 and 6 months (assuming, of course, that he's found guilty).
Yes, you'll almost certainly have to give evidence. The photographs can prove to the court what your injuries were, but they won't be proof of how you came by them. The court will have to hear that from you.
Your post is a little confusing because it refers to a single date but mentions both a 'plea hearing' and a trial. If the 17th March is set as the date for 'Plea and Directions', it will simply be a legal formality (which you almost certainly won't need to attend). Your ex's trial won't take place until some time (probably several months) afterwards.
Chris