I have just successfully cast bt sport from my partners laptop running windows 7! Tried all week and could just get entire screen and no sound but when tried full screen for the video it blacked out. Tweeted btcare who tried to help. Then read a few online articles and discovered this, after clicking on cast icon there's a tiny little arrow in the top rh corner. This gives 3 options. I chose cast entire screen (experimental), that disappears and then chose chromecast. A box then appears asking if you want to share screen and audio output. Simply select full screen mode and its there!
Thanks very much for this information. I have just tried it on my Windows 8.1 Laptop and hey presto BT Sport casts to my TV. The picture is a bit jerky but that is probably down to the pedestrian Broadband Speed at my property. So this now completely blows the "Silverlight" excuse out of the water. I am pleased therefore that Chromecast now appears to support BT Sport casting on both PC and Android devices which gives us customers more choice which can only be a good thing.
However I have found the BT Sport picture being cast from my Android phone using Chromecast has been excellent in recent weeks so I will continue watching it using this method. But at least I now know that the option is there to cast from my laptop if ever required.