Well, if Linux refuses to connect, that pretty much rules out any problem with youe windows installation. There is a hardware problem with your modem or cable, or a problem somewhere on NTL's network.
You need to call CS again and explain to them exactly what steps you have taken. If they refuse to send out another tech, then ask to be put through to someone who can action your cancellation. You don't deserve to be played about with. That said, it's amazing how compliant they can suddenly become if you threaten to walk.
