OK, I have fixed it (in case anyone is interested). I needed to reconfigure the BT Voyager 200, but as it was not visible via the USR8200, I had to connect it directly to a PC. On the Configuration Menu (IP and DHCP), I reset the IP address from 192.168.1.1 to 192.168.2.1. I applied this, and it took a while to recover. Then I connected the USR8200 between the BT modem and the PC again. It worked - straight from the manufacturer's defaults! The only adjustment I have made since is the date.
The Voyager is no longer accessible at click here because this remains at 192.168.1.1. So I have changed my link to 192.168.2.1.
So, it seems to me that there was an IP conflict that I did not have with my D-Link 614+. I guess that was 192.168.0.1 and the USR8200 was 192.168.1.1 - the same as the default for the BT Voyager.