@Wayne-TP   YES that works well. Thankyou for the explanation. 

Also tried changing the action to from "Turn On smart plug" to "Send notification" instead. That way I get a reminder to turn the a forgotten smart plug back manually on which might work better than having it turn on automatically. It is running an electric fence on a farm!!

EDIT: Although I notice the notification still gets sent after the x delay, even if the device has been manually turned back on during the interim. So I need to word the notification accordingly. Instead of Notification "Device has been left off!",  "Device was turned off, did you remember to switch it back on?" .

I gather the Smart Actions belong to each user within their app and cannot be shared to others who have access to the device.  So I need to create similar rules for each user on their respective phone apps? 

If someone turns one of the switches off using the buttons on the P300 Smart Strip, can an automation be set to see & react to that?