I tried your suggestions. I even did a complete factory reset of the Roku streaming stick. The problem remains with Youtube and I notice that Amazon prime video also has the same issue. Netflix and BBC have no issues.
When I load the youtube app all seems ok. I can search and see all the thumbnail videos. I can select them but when I try to play I just get the rotating circles. On Prime Video it's similar except I can load the channel and browse through the various lists. I can select one to play but in this case it seems to load but but no video or audio but I can see the progress bar moving as if the video is playing. I can even skip ahead on the bar but still no video or audio.
I have an Amazon Fire stick on the same tv and there are no problems with it. I have two Roku sticks elsewhere in my house and they both work fine.
This afternoon I swapped the Roku stick for one from another room that I knew was working perfectly. The second Roku stick behaved exactly like the first one in that it won't play youtube or amazon prime video. Therefore I presume the problem lies in my setup. It tried plugging the Roku into a different HDMI socket but that did not help.