how to replace diaphram on a compact 80e?

It has been leaking for some time now and for the last month or so the boiler keeps cutting off when you run the hot water. When the heating is on there is no problem at all. The diaphram cover is corroded and if you take the microswitch off you can visibly see water slowly leaking out. I cleaned out the microswitch with wd40 put it back on and it now works fine so the problem must be here. I have found the diaphram and cover assembly for £30 delivered. How do i stop the water from going everywhere when i take the cover off, and is there anything i need to know when changing this .Many thanks Gareth