I fear this reply might elicit an “of course that’s how it works” response!
The latest is that Humax customer support suggested looking at the Audio settings but didn’t actually suggest what I might do with them. I am even more convinced that the original and subsequent problem stemmed from an HDMI-CEC issue with the Humax effectively cutting the audio and video loop-through and turning this off didn’t help.
However, I decided to try the HDMI-CEC option again and switched this on on the Humax as well as all of the various CEC settings on the TV and tried this again. Previously, I thought that the TV remote would be the one that would be able to control both the TV and Humax, but I was wrong! The Humax remote, with CEC activated, switches on the Humax and the TV, establishes the correct source on the TV for the Humax’s HDMI port and allows the audio to be switched from the TV speakers to our Sonos Beam and connects the video signal from the aerial to the TV. The only time we need the TV remote is to adjust the volume of the Sonos system, which we are OK with.
We are happy to continue on this basis as it would seem to have corrected the video stuttering problem and resolved the other problems encountered after the factory reset.
Thank you again for your help.