Hi
"My name is James and I'm an ntl tech..."
That's certainly an odd one you've got there... And I can see why the first conclusion was "firewall" - but if you've not got one, then... Hmm...
I don't think it's going to be a cabling issue - you're unlikely to have a connection at all if that was the case; it's more likely to be a PC-based, or modem-based issue.
Are you connecting through USB or Ethernet? and can you try pinging out for me? Try
www.ntlworld.com, or
www.bbc.co.uk. Also, what IP address are you getting on your PC? Just the first and last octets are all I need - should be something like 62.253.162.30, in which case all I'd need would be 62.x.x.30. If you're not sure how to ping, or find your IP, post back.