Do you have Simplink (CEC) turned on in your TV settings?

Yes

Do you have any other devices connected to your TV?

I have auto device detection enabled, and also SIMPLINK (HDMI-CEC) and Auto Power Sync.

The only other device connected is a Hyundai box for delivery of TV stations, and it is connected to the TV's DVI HDMI terminal (also via HDMI 2.1 cable), and it works fine with the TV's internal speakers.

I have DTV Audio Setting on Auto, and Digital Sound Out also on Auto. HDMI Input Audio Format is Bitstream.

Disconnect the Hyundai box and the Arc from the TV. Re-run TV Setup in the Sonos app and don’t connect the Arc to the TV until the Sonos app instructs you to. Once the Arc is connected successfully, reconnect the Hyundai box to the TV.

ALSO:

Set Auto Power Sync to Off.

Set DTV Audio Setting to Dolby Digital Plus.

Set Digital Sound Out to Pass Through.

Many thanks GuitarSuperstar - my Arc and Sub Gen 3 are now connected to the TV. The fundamental issue was that I had connected the cable between the Arc and the TV in advance (and also the fact that the cable connection into the back of the Arc is a bit flaky as the cable is quite thick and has to feed through the back of the Sonos Arc wall mount - the slightest movement of the cable and it disconnects).