The thermostat on immersion heaters lives under the plastic cap, where the electric lead goes in. They only need to be disconnected and they pull out from the housing - no water disconnection needed. PLEASE make sure all of the electric is off before you venture in though! It could be the heater element itself however, in which case the water needs to be drained down and the complete unit unscrewed from the tank using a special spanner.
John, January 2008
