Username and password for forms are stored in the browser. Unless the URL or field names change then it should work unless you are on a secure page or have selected the do not save option previously. If you are going through the new website login you could try going straight to
http://webmail.ntlworld.com but if the problem is with previously saved details then click into the usename box and before typing anything press the down arrow on the keyboard, highlight each stored username and press the del kep on the keyboard for each.