You don't need to contact CS to change your username, use this link
https://autoreg.autoregister.net/cgi-bin/chguid (also available linked from the ntlworld.com site). Changing the username, changes the email addy of your master account too. For a little while it might get some emails from your old username account forwarded, but those soon die out. It worked for me a few months ago.
Just remember when you choose a new account name to have something obscure that the "guessing dictionary scanners" spamming programs won't expect. A few odd characters in the name will work.
Having set up that account, don't use it for email. Always uyse one of the other 14 email addys you can set up, so changing those, or deleting them don't affect your master account.