A brief update for anyone who suffers similar symptoms.
As I've said before, I'm using a USB Nintendo style game controller as a pendant with the UCCNC built-in Xbox 360 plug-in. UCVCNC is running on a ten year old HP laptop with Windows 10.

Today the machine was sitting idle for several hours with no problems and then had a shutdown while idle again after an hour-long run of cutting. Interestingly I'd had a Notepad document open over the top of UCCNC for about 30 mins or more when it happened. Later, some while after restarting but not touching the machine, I found that whilst everything seemed normal the pendant would not work. Jogging direct from UCCNC was fine. Unlug/replug the pendant made no difference but closing and restarting UCCNC brought the pendant back to life.

I've since disabled "Allow Windows to stop this device to save power" on all the different USB controllers listed on Device manager and have disabled 'Auto suspend" when plugged in on the USB options in the 'Power' settings menu. I'm still thinking it's a USB problem related to this plug-in and controller and will be putting a real CNC Drive pendant on my Christmas list.