Presumably the Webstar modem also has an ethernet port? My Telewest modem is Motorola but I understood that all Cable Modems support ethernet.
That being the case, you need to power off the Cable Modem and leave it without power for two or three minutes. This is important so that it "releases" the association between the existing connection (recognised by its MAC address) and the IP address it has allocated to it.
Connect the Belkin router's WAN port to the Cable Modem's ethernet port and the PC to one of its LAN ports. Power on the Cable Modem and, once fully booted, power on the router. When that is also fully booted, power on the PC. The Cable Modem should have recognised the different connection and allocated a new IP address to it.
By entering the address 192.168.2.1 into your web browser, the router's Setup pages should open and you can configure the settings.