Apparently the issue wasn't the damage; it was that I hadn't reported it (because I didn't know it had occurred). This breached the contract, so voided my insurance cover.

Now that I've detailed over the telephone and in an email my version of events they've emailed confirming that they've closed the file.

I suppose the scary thing is that they went straight to issuing the claim rather than simply asking me about the damage at any point in the last 4 months. Also I'm fairly confident the car was just fine when I returned it, but that's a moot point now.