To add to Nick's answer - which was a real help to me - I hope he doesn't mind me using his text and adding my small but valuable comment:
At first there doesn't seem to be any way into the switch, but… it does dismantle by first removing the yellow button. It pulls up - but there is a barb on the side of the button nearest the main housing (flatter side of the button) which needs to be depressed first. If you can get a thin piece of plastic to slide down the side of the button, you should be able to pull the button up with something like a mole wrench.
Paul's added help: Instead of plastic, use a thick-ish feeler gauge to slide down, but do it between the grey handle and the grey switch housing - I found (to my cost) that if you push it down alongside the yellow button, the tab or barb on the button will break off. You can depress the tab inwards towards the switch, and using aforementioned mole wrench, gently wiggle the yellow button off. Hope that little tip helps.
Once inside, there is a star shaped screw (need a special bit) which undoes to release the switch housing. Make a sketch of the wiring so you can put it back the same way!
Lever out the brown switch. Pull the connectors apart and resolder the mains lead. Reassemble (as per the sketch!).
Its well worth making sure that the rubber cable grommet thingy goes back in place. I used some silicon sealer to 'glue' it back in place. This should help stop it breaking again.
Hope this help - good luck!
Paul Baldwin, April 2004