dont quite understand in wot is going on but it sounds as if the microswitch could be faulty and there fore creating a demand, therefore rads getting hot. disconnctct lead on microswitch.
Does the pin push the microswitch and thus starting the heating and pump? ( just trying to get my head around the basics of it, but that would make sence! ) if so, wouldnt disconecting the lead just mean the heating wont turn on?
if not, what else pushes the pin from the divider and activates the microswitch as soon as the power is turned on, and the dial set to rads and water?
( confused myself now :s )