Hi Glyn,
You can change your primary username (and thus your main email address) from this page:
http://www.ntlworld.com/help/custome...rPassword.html - this should also change the username on your webspace. If you want to keep your webspace at the same address, I'd say just create a new email account and leave the old one to fill up - once it gets to 10MB, it will start sending rejection messages.
Regards,
Jimbo