I was thinking the TS is in the UK. Or some other 220 volt region.

 

Hi guys, sorry I didn't realise that this thread had so much activity. I'm located in beautiful Australia thus 240v AC.
I know how to wire it up with a relay. The main question was whether or not a relay was the best form of isolation in this day and age or would some other semiconductor be a better option. I'll stick with the relay, it's already been ordered.

FYI:
The 240v feed is from the same lighting circuit. I won't be creating any dangerous loops.
The smart switch will be a Sonoff mini
I don't have any room within the wall cavity to run additional wires from the switch area. I'll be tapping power from the outdoor light plus the constant 240v from behind another switch.

I'm all sorted now. Thanks.

 

The relay is simple and tried-and-true, I would stick with that.

 

Certainly getting more wires into a finished and painted wall is a serious challenge, often made much worse by those who insist on tightly securing the wires with a staple every two feet.