I've already tried the cloning of the MAC address, so I tried doing a hard reset of the router, and it didn't work.
I guess that just about settles it. The router is going back and I'm either going to try another one, or else I'm going to investigate how easy it is to feed the cable through from upstairs to downstairs and connect the set-top box and PC up permanently via the cable I've been dangling along the landing, down the stairs, across the lounge :-)
Thanks for all your help.
Al