Phone NTL support if you want the official answer (at a cost of course) the settings they gave me were:
Under Control Panel - Network setting enter DNS servers 194.168.4.100 and 194.168.8.100
In Internet explorer under Tools, Internet Options - Connections LAN settings enter Proxy server of 213.107.224.4 (colchester primary proxy or 213.107.224.5 (Colchester secondary proxy). Both use port 8080. (Uncheck Autmatically detect settings)
In Netscape it's under Edit - Preferences - Advanced - Proxies.
This worked for me for a while but it looks like my Internet Explorer is a bit shagged cause Messenger says it cannot now find a connection and IE connecting to MSN.com reports DNS server error when I try and open an email. However Netscape Navigator works a treat.
I have been messing about with Naviscope which installs it's own proxy and messes about with Internet Explorer so this could be my current problem as I know you have to disable Naviscope to access MSN Messenger.
The relevant proxies for NTL areas are listed in this sites help pages.
P.S. I run Windows XP so location of settings may be different for other Windows versions.