Are you absolutely sure that nothing has been changed?
Have you tried using a direct connection from XBox to modem?
http://www.cableforum.co.uk/board/showthread.php?t=715
Try that. If that works, then it must be something with your set-up.
I don't think NTL are blocking anything. I don't use the NTL Broadband & COnsole service, & I can still go online (played Pandora Tomorrow a couple of days ago). And I know quite a few other people on XBL with NTL who don't use the Console service, & they haven't had problems.