the information that you have received is correct- ntl can no longer create/ modify email accounts that are have usernames of less than six characters in length.
Can you login to the account at
http://webmail.ntlworld.com ? if not it will say "mail server login failed"- check if just above this it says "unable to connect to message store".
If it is a problem at ntl's end, i'm afraid that there is nothing that can be done to fix this- a three letter username will just not be allowed to be provisioned on the system.
If you want me to check the account from the ntl side- PM me the username and i'll have a look at it tomorrow.