That sounds way too drastic (although it is pretty certain to work, if the problem is at your end).
For now, trying uninstalling your network adaptor software (if any) from Control Panel > Add/Remove programs, then uninstall it from Device Manager.
Power off, physically remove the adaptor, power on.
Power off again, reinstall the adaptor - that will certainly release your IP address.
When you first try to connect to the Web, you will probably be taken to the NTL signup pages again.
Just enter your existing username and password, close the browser, and restart the browser.