How to Claim (in theory) for Lost Royal Mail Returns

Right, Amazon still haven’t actually provided an update regarding all things lost FBM Amazon returns but we’ve had our first instance of a lost FBM return via Royal Mail where the customer has used the prepaid Amazon Return Label.

Just wondering if anyone has any experience of claiming, and what is the workaround to actually claim? I went to the Royal Mail Claim Centre but not clear what route I’m supposed to follow!