@jkandasa Sorry... I thought I'd edited or deleted that reply saying it didn't work. It now works... I'd taken the sensor ID from the summary page and not the edit page as you pointed me to in your last reply.
So its working now... my mistake, thanks for your patience.
This is excellent news - now even more control from within myc !!!
There are so many uses for this flexibility - I only asked for a reboot/shutdown button as it would eliminate an ssh connection each time I need to work on the pi - but this is just way better than that!