Assuming you are using ntl
Click this link to the ntlworld website:
http://www.ntlworld.com/helpsupport/...ail_manage.php
Check the change username option. Changing that should, once the servers get to reset, update your master email address. YOu'll then need to change that in your otulook settings too.
Remember this changes the master email, and username for dial up type logon. Your ntlwebspace will retain your initially chosen username and the ntl allocated password.