Thanks guys for the replies. And please do forgive my absolute lack of knowledge on all this tech stuff. I feel like I'm fast becoming one of those people who struggle with all this new tech stuff!
Right, so an Apple TV in 'stand-by (where the red light remains on like on DVD players, TV's etc) can and will allow me to switch lights on/off and control my heating on Apple HomeKit products when I'm at work, at a friends, partying (yeah right!!!!) etc ?