Under the normal court processes, the fact that your ex was on probation can't influence the jury either way because they can't be told about. Normally the prosecution are not allowed to make any references to previous convictions.
However, it's possible that the Crown Prosecution Service might make an application to the court under the 'propensity' rules. If the CPS can convince the judge that your ex's previous convictions show a propensity to commit the type of offences with which he's currently charged, the judge can rule that his previous convictions can be made known to the jury. (When these rules were first introduced, they were used almost exclusively in relation to sexual offences. However, the CPS has increasingly been successful in making 'propensity' applications in respect of physical violence).
Even without a successful 'propensity' application, it's worth remembering that (in order to save the public money) the Crown Prosecution Service will not take any case to court unless they believe that a conviction is likely. So, their experts have already decided that, in their opinion, there's sufficient evidence to convict your ex of these offences.
As Shadow Man indicates, jurors are unpredictable. Many of them will follow the evidence carefully and come to the correct conclusions. However, idiots like Cambus have their own (extremely alarming) system for determining guilt or otherwise:
http://www.theanswerbank.co.uk/Law/Question464 060.html
However, based upon what you've written, I would think that the CPS stand a very good (almost certain) chance of obtaining a conviction. Given your ex's previous convictions, I'd estimate that he'd receive a sentence in the range of 3 to 7 years imprisonment, with the upper end of that scale being more likely.
Chris