In reply to NaCl:
Could you, via agreeable conversation with your neighbour, find out whether he's in the same city still, or has moved to another location?
I got some mail for a fine for littering for somebody who lived elsewhere in the city because she'd given the warden my address, and I googled her name (and city) and 192 came up with a partial address for her, with me needing to pay to view it all. I didn't pay but I wrote to the council and explained she didn't live with me, and two council enforcement blokes turned up and they took me at face value that she didn't live at my address (talking about the last 7 years it only having been me, and a couple before then seemed to help).
If you could find out his location, and then 192 his name and where he is, you might be able to get an address, and possibly a number too.
I'm not too sure what you'd do with his details if you could 192 them, it's my personality that I possibly might knock on the door and go 'Aha, it is you' and give him his mail, and forward the new address to the credit card company, but I don't know how legal that would be.
Edit: You wouldn't be doing anything morally wrong, though, and he's possibly not being legit - being the only one who could complain. If there is a number, you could ring and see if he answers too, which would be more chilled.
Post edited at 14:27