Sorry this has happened to you.
I'm wondering if the explanation is that the employer isn't relying solely on the police case, (which sounds like it went pretty far if you got summonsed to court). The CPS in a criminal case have to prove their case "beyond a reasonable doubt"which makes it quite difficult for you - even though it was your IP address, they don't seem to have been able to prove it was you at the computer at the time, so wouldn't meet the burden of proof required in court. This doesn't necessarily get you off the hook - they're not saying it wasn't you , they're saying they had insufficient evidence to bring the case.
However your employer can determine a grievance on the lower burden of proof required in the civil courts - which is "on balance of probability". Basically, that when looking at the evidence, a reasonable person would believe there is a 51% or more chance you were the responsible person. This may be much, much easier for them.
If it comes to it, I would expect them to show you a list of times when these complaints were made, and you in turn to provide some account of what you were doing at these times. If you can't account for your time, or you can't account for enough of those incidents, you might have a problem. How would they find this info? At my work The IT dept can give reports of who was logged on when, what applications they used, keystrokes they made and websites they accessed at any given time. Or your employer might try to demonstrate that, on balance, only you had access to that IP address. The allegation of a colleague purely on the basis that the police identified your IP address wouldn't be good enough for me to determine it was you without further evidence.
By the way my job involves investigating and preparing civil cases against people.