First of all, many thanks for taking the time to respond.
To answer your questions:
1. Yes, this email account does show up in the "Manage other Virgin Media accounts" section when logged in under my wife's account
2. When I try to add my email account to the web mail interface, I get the error "The provided password for the e-mail address *****@ntlworld.com is incorrect". (This was previously working, but I removed it and tried to re-add it when this problem arose).
Something has clearly change but, as I say, I'm not aware of any changes at my end. Suspect a call to Virgin and a password reset will be required, but keen to hear your thoughts.
Thanks again for taking the time.
NB