>>>He however believes he is not guilty as we were arguing prior to the indecent, and says this means i provoked him
That's no defence whatsoever. You could have called him every name under the sun (using every swear word in the dictionary). You could have insulted his mother, you could have thrown a hammer at his head, you could have kneed him in the groin. Even if you had done all of that, the moment that he touched you (even if it was only a simple shove) he would have been automatically GUILTY of assault. (The only defence would be if he could show that he had used the absolute minimum force necessary to prevent YOU from attacking HIM, which is clearly not the case in the situation you've described). 'Provocation' can be taken into account when SENTENCING an offender but it is most definitely NOT a defence!
So there's absolutely no doubt whatsoever that he is most definitely GUILTY of some form of assault (irrespective of whether that's 'Common Assault' or 'ABH').
For the prosecution to convince the Court that he is guilty of 'ABH', rather than 'Common Assault' they will need to show that your injuries were 'serious'. It is up to the Court to decide what level of injuries fall within 'ABH' but the Crown Prosecution Service guidance advises that injuries requiring stitches (but not just the application of steri-strips) constitute 'ABH', as do injuries requiring hospital procedures under anaesthetic. So, for example, an incident resulting in a few bruises may well be classed as 'Common Assault' but a broken nose would probably constitute 'ABH'.
When sentencing offenders for ABH the Court has to consider both 'harm' and 'culpability'. Your ex, or his barrister, might be able to convince the Court that, in view of the alleged provocation, 'culpability' was low. If the Court is also convinced that 'harm' is low (i.e. your injuries were at the lower end of the scale for 'ABH' to apply) then it's a 'Category 3'offence and the Court can't impose a custodial sentence. However if 'harm' is assessed to be 'high' (while culpability is 'low') it's a 'Category 2' offence and a custodial sentence (probably of around 6 months) is likely but not certain.
If your ex, or his barrister, can convince the Court that 'Common Assault' applies (and assuming that 'culpability' is still 'low' through excessive provocation), only a non-custodial sentence can be passed.
See here for much more detail on sentencing:
http://sentencingcouncil.judiciary.gov.uk/docs/Assault_definitive_guideline_-_Crown_Court.pdf