If NTL is the same as blueyonder/virginmedia then you get a username and password in a letter when u join, its a good idea to keep this letter safe just incase u forget. Both user name and password are a combination of random numbers and initials and random letters. The primary email uses your user name and password by default. Then the 2nd email uses your username_1, password can be changed in selfcare.blueyonder.co.uk; but u need to know your user name and password.
The email can have 3 alias', this can be a name u make up yourself (also done in selfcare).
If u dont know your user name and password you cant login to selfcare and so you are out of luck. You need to ask them what it is.
If you are trying to use an FTP program and you have an FTP storage connected to an alias, u should be able to log into your FTP storage space using your alias and password.
If none of that made any sence then u still need to ask NTL what your user name and password is, and then keep it secret too.
