I have an insurance policy that covers excess on car hire world wide. I did have to use it once about 4 years ago, hired with Eurocar, damage was noted upon return, I paid around 300 euro, went to process to claim back and got refunded no problem.

Recently, again when returning a hire car in Europe, damage was noted and I had to pay 400 euro. Submitted claim, they acknowledged recipe of all relevant documentation and started to process claim. They then said that they had tried to contact the car hire company to obtain a "damage return document" and the rental company are not replying to them.

And that's the state of play at the moment.

Any advice how to move forward?

Which insurance company?  Which rental service? Which Claims Service do they utilise?

Looking at their T&C might help to paint a picture.