Hi @PaddyB. I hope you'll be able to help me with a similar problem.
According to my bill, I have BT TV App Extra. However, it will not stream to my Chromecast.
If I try to stream from the BT Sport App, it works with no problems. However, if I try to stream either live TV or Catch Up from the BT TV App, I get an error message saying I have to upgrade to App Extra. I double checked my bill and I do have it.
After reading this thread, I called BT support on Tuesday but they were unable to solve it