It will look like this. I don't know what color wires you use in UK but the switch will have two on it now. Add a three wire cable from the existing switch to the new switch and hook it up like this. The two wires coming from the light fixture in the drawing are your existing two wires on the switch.
A forum community dedicated to Do it yourself-ers and home improvement enthusiasts. Come join the discussion about tools, projects, builds, styles, scales, reviews, accessories, classifieds, and more! Helping You to Do It Yourself!