Ken Knott wrote:
i have just fitted a resettable immersion heater thermostat. It has a
dial to select the temperature and a reset button. Since you can
select the temperature using the dial i don't really understand what
resettable means in this context. can anyone explain? thanks.

In case it gets too hot for some reason, you can press the reset button
when the overtemp thermostat cuts it out.
Otherwise for one with a thermal fuse, you'd need to replace it.