Isolate light circuit - remove old switch fit new switch - restore power. If new switch has Com, L1, L2 terminals put one wire in Com and other wire in either L1 or L2 as you wish.
If no switch to begin with - dig hole in wall for cables and metal (pattress) box - run cable from switch position to ceiling rose position. Cover cable run and make good wall - wire up and install switch & lamp as required.
Peccavi, June 2009
