See the answers from Steve W and Steve D. Follow their instructions and you'll sort it. It's VERY fiddly and frustrating but I got there in the end (after about 3 hours of trying to locate the plastic switch retainer down the side of the switch)by following the method of drilling into the side casing. It sounds more drastic than it is as the holes are on the inside of the casing so can't be seen afterwards.
Richard M, April 2008