Immersion Heater Problem

Hi,

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)...

Regards

Gary