The On/Off button is secured by a lug. There is a tiny tiny hole at one side which allows you to insert a small flat bladed screwdriver (a jewlers screwdriver is ideal) to release the lug from it's retainer (lever it towards the centre of the switch). Then you can withdraw the button upwards. It's very fiddly!!
With the button removed, the switch and switchholder is secured by a 15T star headed fastener - unscrew this if you can and the whole assembley slides out the top.
Malc B, November 2008
