[Halcyon]

Yep, as Hom3r says, a router is your best option.
Even if you do contact NTL for an additional modem, you will be charged for having 2 broadband services due to having two modem connections.

A router plugs into your NTL moodem and then you can connect each of your computers to the router.
You can either do this with Ethernet cable. It is cheap to buy and run it from the router into the back of your PC's ethernet card.
Or.... Buy a wireless router and stick a wireless card in your PC.