Struggled with this for ages - and then the answer is very straight-forward. You don't even have to push something down the hole if you have strong fingers...
Simply push hard on the curved edge towards the handle - as you do this, push upwards and the top of the button simply lifts out as the clip is compressed. The screw holding the switch assembly is can then be accessed - cross-head screwdriver needed.
Iain Humphries, August 2005