Atmos solved - YAY!
So finally I get to see the longed for 'Atmos' displayed on the front of the Yamaha RX-A3050, actually playing content from the AppleTV. Anyone guessing it was the AppleTV causing the problem can pat themselves on the back. But the solution was not what I would call obvious.
First of all, I totally failed to make any headway playing content from a USB stick in either the A3050 itself or my Blu-Ray player. Both have real problems with anything 'unusual' and simply refused to play ball.
As suggested, I swapped the HDMI cable from the AppleTV to the A3050 to a longer one from a Sky Q setup we no longer have. Since Sky Q can play 4K and Atmos, I figured it must be capable. But no difference. Still no Atmos.
So I swapped the Apple TV as I have another 4K one elsewhere and 2 of them cannot both be faulty. Well, faulty may be the wrong word, but they both exhibited the same problem - no Atmos. Until…
Looking at the Apple TV setup, in Audio and Video section, the Audio Format is set to Auto and in that main section screen, displays 'Auto', even when you enter that option and see 'Immersive Audio' is enabled.
Then while looking at the settings thinking I wish I could simply select 4K and be done with it, on a whim I tried changing the video from 1080P/50 SDR -> 1080P/50 HDR. I don't believe my Panasonic plasma is actually HDR, but what the hell, might as well try. If it doesn't work, it will revert back so no problem. However although it did actually produce a display on the TV, the AppleTV menus and screens look really washed out. Dreadful picture in fact, but not expecting much I tried the Dolby demos again…
Yay! The A3050 finally actually shows Atmos and all is working. I'm not sure if the picture quality is actually worse, like the AppleTV's own screens, but the all important Atmos is now functioning. So…
You may imagine from this that the AppleTV requires the video output set to HDR before Atmos will function and indeed when checking the Audio and Video section of Settings, Audio Format now shows 'Auto, Atmos available', so it has definitely now got its act together and all set to work with Atmos.
However, this was all with the other AppleTV that I was trying and that had to go back to another room with a TV that is definitely SDR, so I switched back to 1080P/50 SDR and was rather surprised to see that Audio Format still said 'Auto, Atmos available' (but much more clearly now set back to SDR) and when I again tried the Dolby demos, they still play perfectly with Atmos.
I then swapped back to the original AppleTV for that room and changed to 1080P/50 HDR resulting in the same washed out AppleTV screens and Audio Format now showing 'Auto, Atmos available'. But I then immediately switched back to 1080P/50 SDR and lo and behold Audio Format remains as 'Auto, Atmos available' and the Dolby demos play perfectly with Atmos.
Apart from my obvious joy and relief to get Atmos working, I am completely bemused by this. In order to actually play Atmos, the Apple TV 4K must have been switched to an HDR video mode, even though it can then be reset to SDR. This is totally bizarre. First of all, why in any way link those 2 features. The sound quality has NOTHING to do with any video format or quality setting. Secondly, why will it refuse to work on SDR until HDR has been selected, enabling Atmos but then still working once switched back to SDR.
Needless to say, thank you to all who assisted, but I'll bet NO-ONE expected this to be the cause. However, if anyone can shed any light on this bizarre AppleTV behaviour - please do.