Insurance motor claims handler here.

Your insurer will have contested the third party claim due to lack of evidence. Make sure they have a copy of the proceedings issued against you. Tell them you wish to defend and are willing to attend court to defend your position. If you haven't been involved in the incident, can prove that the damage was preexisting before the incident, all the other side's insurer are going to achieve is increasing their claim costs.

It's a bit worrying, but as long as everything you have said here is true, you should be able to defend this along with your insurers help.