Thanks for your reply Ailbhe,
I could argue that the source of the rust is the original damaged area and request that it be made good under the policy.
The rust area is smaller than a 1 cent coin but the original damage may only have been the size of a pinhead and was not noticeable after the accident due to its location and size. I couldn't reasonably take any steps to prevent loss or damage when it was not noticeable. Even fixing the bumper immediately would not have prevented this occuring, again assuming it is due to the accident. Worst case scenario I'll pay for the the consequential part if they pay for the original part i.e. go halves on the cost.
My query relating to the time limit in general still applies as I'm not even sure if the bumper repair is within such a limit and don't want to press the issue until I'm more confident of my rights.