[MovedGoalPosts]

This is weird, I'm not sure that I understand exactly how you are wired up.

Can you post the exact model number of the modem, just so I can be sure what you have? IT's the STB connection that confuses me. The modem itself is connected directly to the Telewest network? It's not got any other box of tricks wired in elsewhere?

All ethernet cables should normally be straight through. You can verify that both work OK by trying a simple connection between laptop Ethernet and modem ethernet. If that works, swap the cable for the other one and confirm that works too.

It'll be worth doing a full factory reset on the router (manual should say how) just to be sure all is at default settings, then reboot everything.

If you are repowering the modem / router / laptop between cable swaps, and you still cannot get a connection between the router and modem, I've got to suspect something iffy with the router's internet connection.