Thanks for this. NeilO has been in contact and is taking the matter in hand - so hopefully it will all get sorted now.
Thanks again to both of you for your help.
Similar issue for me. I had the BT Sport app previously with a broadband package and could cast via Chromecast.
We switched broadband suppliers but I took out a BT mobile contract because it gave me BT Sport via the app. I assumed I would be able to case and don't recall seeing any information to the contrary.
I now find I can't cast and this appears to be slightly underhand in that there was no clear warning regarding this.
Has anyone found a way to adjust one's subscription to obtain casting functionality? If not, I might just quit the contract at this stage.