Never hear back from 3rd line support

Cant see your screen shot, are you logging on to the virgin media section of the sky app, little vm icon below the sky login. or 

 I think you can delete your Sky devices and re-register them by using the TV GO app (I use a laptop), it's lot of messing about!!

Log into the TV GO app, click your profile and choose Settings, then find Device Management near the top of the screen and click it, you should see an option Manage my Sky devices at the bottom of your listed Virgin devices so click that and log in to see your Sky devices. You can then delete any you don't need, I had all 6 slots used so deleted them all and ran the Sky Sports App on all the devices I wanted to register them on. This also depends on how many device changes you can use for the particular month you are in, it should tell you how many device chages you have available when you log in!

Also bear in mind you can register 6 devices for Sky Sports but I think you can only watch it on 2 devices at the same time!