BT Sport seems to be missing when I try using Virgin TV Go, signed out & back in with no change, at one point the app seemed to suggest that I needed to upgrade in order to watch, but it should already be available on the package I'm on.
The channels seem to be back for now, but just to answer your questions -
"I understand at one stage the app suggested you need to upgrade - do you get any other error-messages at all?" - no others from what I could see.
"Is it all of the BT Sports channels or just one in particular?" - all BT sports channels where missing.
"Are you able to view those channels via your TV box as normal?" - didn't try it through the TV
"Are you in the UK when it happens?" - Yep
"Please also let us know the device/browser(s?) you've tried." - Tried using the app on my phone (android) tried using the app on my pc & tried logging into view via the virgin media web site through both chrome & firefox.