It looks like quite a few people have seen their email accounts hacked and passwords changed, then changed again. I believe this isn't helped with the sessions on browsers keeping the other person signed in even after they have changed their password.
What might help, if you can access your email, is go to the settings in the upper right (cog), goto security, then set Automatic sign out to 5 minutes. Ensure forwarding isn't set to another email address.
Then, if you can, go to the virginmedia account and change the password and security question. Hopefully, if anyone is manually doing this, they should be signed out before they can change any settings.
I still can't change the phone number on the account which is worrying, but my password has remained the same for the past hour. Hopefully someone is working on this... and will implement 2 factor auth.