You don't say which type of cable modem you are using.
You may be able to see what service you are on by going here
http://192.168.100.1/ & clicking on "operation configuration". The downstream & upstream data rates will let you know what speed you are on.
Username & password = root.
Has your download speed always been slow since getting ntl broadband? If so, it could be due to windows RWIN value being set too low. If it is, the link in Ignition's post will sort it out.