Well I have enabled WP DEBUG and see no errors appearing on dashboard or front end. I changed the theme over to ‘Twenty Sixteen’ and the inspector error message disappeared.
It came back when I re-activated the GP child theme again. I also tried just activating the parent theme and the same error message appeared in the chrome browser inspector.
Note: Chrome and Firefox give me the same inspector errors, MS Edge gives me all kinds of weird errors (but that is hardly surprising coming from MS anyhow). Unsurprisingly Opera browser does the same as Chrome. I have not yet tried Safari but being webkit, I’m like 99.9% sure of the result.
I think my last resort is to setup a test install on another server somewhere….