I've now received a brief reply from Gigabyte support regarding the problem I started having with XTU's watchdog service after updating Control Center and re-installing XTU:
"Please kindly note from the Intel XTU site, the utility is not valid for the laptop CPU. (You may refer the Intel's site
here) In addition, the setting in Intel XTU will force to change certain settings in the Control Center that we do not suggest you use the utility on the laptop."
I don't understand a word of it - everybody seems to believe that under-volting is recommended for laptops of this type. I had the same CPU in my Lenovo X1 Extreme and XTU worked very well. And what are the 'certain settings' in Control Center that get altered? Also, this reply seems counter to the response offered via Atom-08 earlier when no mention of unsuitability was made. I am thoroughly confused now - XTU appears to come bundled with Control Center.
If I can't under-volt this laptop then it's not much use to me - and of course I am now several days out of my return period.