I found that with Mandrake 9 (and I think RH8 should be the same) that with my network card and NTL cable modem turned on during install all the connections were made - if you have done the same, you should be ok. You may also want to try the forums on
www.nthellworld.com - a couple of users there have step by step instructions for connecting to NTL with RH8.
Good luck.