The lighting in your house is looped from lighting point to lighting point, i.e. room to room so you will have a live and neutral coming in (one cable) and a live and neutral going out (second cable to next room, if this is not connected to the first cable then the lights in other rooms will have no power). The third wire goes from the lighting point down to the switch(switch wires). Live in and live out connect together, neutral in and neutral out connect together (and to neutral of light fitting). Red to switch connect to live, and black to switch connect to live of light fitting. Connect all earths together. Hope this helps. Wiring diagrams for the above should explain more. http://www.diydoctor.org.uk/projects/replacelightfitting.htm
Good luck
Baz
baz, November 2007