But if its a return, surely they can pair it with the original purchase, inc. the address and card used?

The card might have expired and be destroyed, so you could have a valid reason for not being able to produce the number.

In which case, you would simply get in touch with support and prove your identity and purchase some other way as my thinking is that this would a rare occurence, since receiving a faulty purchase pair using a card that has expired and you no longer hold whilst living at a completely different address is a freak event. Plus, couldn't you still use the expired card as verification even it expired?