On NTL Broadband you will have previously registered the Mac address (this is the 12 character physical address) with NTL. This address is the only one recognised by your cable modem and is the only one which will connect to the NTL server. Have you cloned in the MAC address into your router so that the cable modem still thinks it is talking to the originally registered PC? This is what I have done using my Linksys Befsr41 router/switch on NTL 600k
standalone BB. Had no trouble doing it this way and its been okay for 15 months. Either PC will connect to NTL without the other having to be switched on.
Have a look at robin walkers website for info if you haven't already done so