While the TV audio is playing, select the System tab in the Sonos app (middle icon on the bottom of the screen) and tap twice on the room associated with the Arc.

Thanks, I tried that but it does not offer anything different, with the ‘Now Playing’ bar showing what was paused on a Play 1.

However, I unpaused that, linked what was playing on the Play 1 to also play on the Arc. It registered on the phone that it was playing, but the TV sound was still playing on the Arc!

I then removed the Play 1 device and got the option to select the Speech Enhancement on the Arc. It said it is setup, but not sure that it is correctly as at no time was the Arc playing anything other than the TV, even though it was supposed to be playing a Sonos radio station.

While the above sounds confusing, that is the only way I could see the Now Playing box!

Am wondering if I will notice any enhanced speech after all that.

Reboot your phone. Then make sure TV Autoplay is enabled under the Arc settings in the Sonos app. Turn on the TV and play audio from your TV. You should hear TV audio playing out of the Arc. In the Sonos app, go to the System tab and double tap on the room associated with the Arc and enable Speech Enhancement. 
 

NOTE: The Speech Enhancement setting can only be used when playing audio from the TV. It cannot be used when streaming music from the Sonos app or from any other music streaming apps like Spotify or Apple Music on your phone.

Thanks again. I have rebooted the phone (again), but this time powered off the TV, Arc and Sub also.

Once I rebooted all seems to be fine now, I can access the screen to enable / disable the Speech Enhancement. I will test the actual functionality properly this evening when I have more time.