I'm fitting a new dimmer to a switch that controls a single ceiling bulb. Currently the switch has two red wires attached, one top one bottom. I'm guessing that this is live from circuit box and then live to ceiling rose. The two black wires have been connected together with a small box and the green is attached to the metal fixing embedded in the wall.
The dimmer I've bought says to fit a red and a black to terminals c and l1 respectively. Do I choose any two red and black wires? The instructions mention if there are two wires then put them both into the appropriate terminal on the switch but that seems like it would result in a constant connection between the two live wires.