Interestingly, the problem has gone away this afternoon!
At the same time as testing the backlight setting (which had no affect), I also went to the Home screen, which we don't normally do since we run everything from a Mac via HDMI. I also connected the TV to Wifi (which we also don't usually have connected, since it's not needed).
I don't see how either of these actions could have actually solved the issue and suspect it is just coincidental, but you never know?!
It seems more likely that not using the TV for a couple of days has allowed some component to reset or return to normal working parameters. I guess I'll monitor over the next 48 hours and see if the issue returns.
Thoughts ??