I have powered off my modem and back on when it was plugged into teh Amiga because I read somewhere that it caches the MAC address of the adapter it is connected to, but this will clear if it is turned off for 10 seconds.
Correct.
If you have manually specified an IP / DnS server, forget it, its not going to work. The NTL Modem *has* to be connected to a device that has the DNS / IP side of things set to auto detect.
Once you have an IP adress, open a browser and type 192.168.1.1 and you should get a User / Password prompt if all is OK.
Make sure also you have no proxy server's specified, unless its an NTL proxy, otherwise thats not going to work either.
Kev