Thank you Pablovi77 for your reply. Much appreciated. I've now understood the work-around ... and it works! However, the default audio option (which used to be there) has completely disappeared from my system, with the result that the Apple TV 4K that used to play quite happily through my Homepods stereo pair had lost the ability to do so. And all my Apple stuff is always kept bang up to date.

OK ... I freely admit that I'm not the smartest cookie in the jar, but there's something I'm not getting here with this workaround. Since my Apple TV has updated itself to tvOS 14.2, I've lost all ability to connect the Apple TV with the Homepod stereo pair. There is no 'Default Audio' option under 'Video and Audio' ... it just doesn't exist. And when I press and hold on the Apple TV icon, the only options I get are to create a new folder. What am I misunderstanding? It was all working fine until this update.