For future reference and to help anyone else with this problem, I have finally worked it out, in conjunction with high level tech support.
It would appear that you have to access virgin media services, particularly the sports and movies apps using a primary account, ending with virginmedia.com. My access has always been via my old ntlworld email address, which, in effect, was an alias access (if that makes sense!).
The chap reset my account and I logged in successfully.