Thanks - although Virgin had told me they were linking the email to the account, it seems like the ntlworld email is just being treated as a contact email address, so there's no option to change its password via this route. Can I get Virgin to link the email to the account?
As the ntlworld.com email address was not linked to your active account you should read this post, Orphan Email accounts to understand its status. Mix and match of email addresses between accounts is not possible however the complete set can be moved from one to the other. I have flagged your post to the forum team who are best placed to help further however be aware it can take them a few hours / days to respond.