The "forgotten password" button is a joke - anything it leads to also requires me to enter my PW!!!!!
It took about half-hour of just keep trying things till I got a reset PW link.
The main issue is: myvirginmedia won't let me sign in because I don't have a VM mobile number.
I've followed both links myself and the prompt for the VM mobile number seems to come up when following the forgotten password link for Mobile Account sign-in.
I wonder if you clicked this by mistake?
Navigate to www.virginmedia.com > click on Sign In at the top right corner > select My Virgin Media from the 2 available options (see pic. below).
Hope this helps, keep us updated