I too need help to delete an old ntlword email address which I have not used for 8 years. Every time I send an email from my gmail address, the ntlworld address appears in the 'from' box. It causes immense frustration to people who try to reply and embarrassment to me. I cannot delete it myself as I no longer know what my virgin login details are. I've spent hours trying to work this out but am defeated each time.
I think that you have misunderstood. Once you have deleted the Ntlworld address from your Gmail settings and replaced it with the Gmail address the issue should go away. It isn't something that you need to do each time.
Do you have a current VM broadband account? If so is the Ntlworld email address that you are concerned about the primary username for the account?
What happens if you try to log in to the Ntlworld email address via this link: