I have usually bought third party car hire excess insurance but so far have never needed to claim.
Firstly, why buy it? Otherwise you can be liable for a massive repair cost up to £1500 (or you can take out the hirer’s excess cover at ten times what a third party will cost).
So I bought from worldwide.com where I didn’t have to pretend I lived in the UK €12.50 for 5 days cover. The car hirer wanted £150. I duly checked otu the existing damage to the car but missed a loose plastic grill on the front. So when I returned I had a £480 bill to pay. Worldwide wanted some documents I dodn’t have (like an accident report - I shoulc probably have taken photos) and often the hirers don’t do the repair anyway. I had rung the hirer asking for this and it never turned up - but in any case worldwide settled (only less £25 because the admin by the hirer was £75 and they only covered £50).
So I can recommend Worldwide.com