With the PC connected directly to the modem and online, open a Command Prompt window and run the command:
This should release all addresses and set them to 0.0.0.0
Shut down the PC and the modem. Leave the modem without power for two or three minutes. Connect the modem to the router's WAN port and the PC to one of its LAN ports. Power on the modem and wait until its lights indicate it is fully booted. Then power on the router and once it has booted fully, boot the PC. The IP address that the ROUTER gets from Telewest should be DIFFERENT from that which the PC had. However, although dynamic IP addressing is used, if you reconnected the PC directly to the modem again, you might well get its original address back, unless it has been reallocated to another user in the meantime. I had reason to connect a PC to my modem yesterday, bypassing the router, and when I reconnected the router, it got its old IP address back again.