Immersion Heater Problem

I have had to recently replace my immersion heater. However on the new type thermostats that come with them they have a reset switch. The reset switch is constantly tripping out before the tank has reached the temperature set . I have tried a couple of new thermostats and also replaced again the whole immersion heater (i bought one from bnq then from Screwfix). I read on the little leaflet within that it could be some electrical fault. I am in a hard water area (well so im told from websites ive found) so therfore bought the following (which looks exaclty like the old one i removed)...
where do i start and what can i use to diagnose what is causing the fault. any help really much appreciated.
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