I've had a look at the wiring of the existing two switch dimmer and can't make out what's going on with the earth. There are two earth (green and yelloW) wires coming out of the wall along with the live and neutral wires. The other ends of both earth wires are attached to the old switch casing/box (which will remain), by means of a screw. The new dimmer (homebase) has a specific place marked for the earth to be attached, to the rear of the switch. It does not come with a box/casing. Instructions say the metal plate dimmer must be earthed to the casing/wall box. Do I detach one of the earth wires and reattach to the marked spot on the plate?. Or both? Or none? Help please!