Thanks again for your help everyone.

The replacement part has arrived now 🙂

The seller is quite insistent on me not cancelling the case for some reason, instead just letting it age out.  Eg "And remember don't close the return case by yourself, it will be close by eBay, just ignore it. Don't close it by yourself."

I don't understand what difference it'd make, but i suppose there's no harm in leaving it to age out so i'll just do that.

It got the job done in the end, but if anyone from ebay follows these threads - the process is really unintuitive and could do with rewording/restructuring for when people have this issue, which i imagine is not uncommon.