There is a work around if your ntlworld.com e-mail account has been migrated to the new googlemail platform.
The ntlworld.com SMTP server is only accessible when you are physically on the NTL/Virgin Media network.
The virginmedia.com SMTP server, however,
can be accessed outside the NTL/Virgin Media network!
To enable access for an @ntlworld.com e-mail address:
From the 'My Virgin Media' page go to 'Manage Household Accounts'.
Add a new mailbox with the e-mail address ending @virginmedia.com.
Use this account to authenticate with the virginmedia.com SMTP server, but send the e-mail from your @ntlworld.com address.
So in GMail, for example, you would configure 'send mail from another address' as follows.
First configure the @ntlworld.com account you are sending from (see thumbnail #1).
Then configure the virginmedia.com SMTP server and authentication (see thumbnail #2).
Hope that makes sense.
James