I have 3 outdoor wall lights, one is controlled by an indoor switch and the other two are permanently live but have built in pir's. I would like to replace all three lights and add a photocell so they come on automatically at night. I've looked at the wiring and each light is fed with just one twin and earth cable and since the same switch on the consumer unit switches power to all lights I presume they all connect back to a common point somewhere in the ceiling space. I have no idea where that point will be so will I have to install 3 separate photocells to control each light independently or is there a way I can add just one photocell to control all three?