[wraymond]

Thoughts occur Mick.

Was the renewal at the exact and proper contract date?
Did you actually have sight of the claim papers, or any other indication at the time of the incident, that acknowledged your non-liability?
Have you any correspondence from any source concerning the incident?
What percentage increase of the expiring premium was the new premium?

It sounds to me like a basic rating increase rather than a penalty or liability charge. You can ask, even demand, copies of all correspondence relating to that incident if it affected your renewal.

You can formally request details of current premium rates compared to most recent ones. You are entitled, and they must comply, to enquire about precise details about rating increases compared to last renewal. They'll wriggle, but the word 'ombudsman' is powerful.
Only you can decide if in your case it's worth it!

I would, and for every last penny. (25 years in the biz)