I was using until recently a Netgear router on NTL on Live (it was the DG834 though, not the one you mention) without paying for the second IP (I had 2 PCs and 2 Xboxes running simultaneously actually). I think the thing you need to be aware of when doing this is you may need to clone the MAC address of the first thing the NTL connection sees when it connects, ie. probably your main PC. You can do this in the Network Settings screen of the Xbox dashboard. This might do the trick, it worked for me!
NTL uses dynamic IP addresses, and you cannot therefore set a static address for the connection to the internet. Shouldn't be a problem though, just leave it set to Automatic, except for the DNS which should be set to 194.168.4.100 and 194.168.8.100 (I think these numbers are right - I will check tonight and edit if necessary) in anything that shares the internet connection.