So, after a lot of to-ing and fro-ing I've finally been able to create my user account for web billing etc.
To create the account I ended up choosing the "use virgin media email address" option as my username, create the new VM email address, continue with the register process, and I can now sign in to my Virgin Media account.
Still think it's strange that I couldn't use my non-VM email address as my username/login for VM services, especially as my non-VM email address is still showing as the default contact email address on my Virgin Media account when I sign in to my VM account.