If the water is either way too hot or too cold after adjusting a thermostat on the immersion heater I’m assuming here nothing wrong with the elements and that the thermostat is broken. Can the thermostat be replaced on its own relatively cheaply or does the whole immersion kit have to be replaced?

Yes, electrically isolate the immersion heater and remove the cover. The stat simply clips onto the terminals of the heating element and pulls off (in a strsight line, there’s a temperature probe on it).

You need to elaborate.
After adjustment, are you leaving it on long enough to get warm?
Or is it on all the time?
Or, it is pretty much on or off?

The thermostat (for a least 10 years) is a separate piece, that sildes into the centre of the heater element, and can be changed quite easily.Good plumbers merchants will stock them. Get the right length.

Alanl – it’s a rented house so I’m not there but from what I know the thermostat was adjusted and left on long enough for it to take effect. I think the immersion works on a timer but I can double check. Looking back at the records the elements were only replaced in February of 2015 so surely the thermostat can be replaced separately.

If the water is either way too hot or too cold after adjusting a thermostat on the immersion heater I’m assuming here nothing wrong with the elements and that the thermostat is broken.

If the thermostat dies the water will not heat at all. On our Megaflo the thermostat has reset button that can be pushed back in if it trips.

Given I replaced the immersion elements in Feb 2015 I questioned the maintenance companies call to replace the full immersion unit and tried to see if they could replace the thermostat only and this is the response I got: (of course the elements were only covered for 12 months labour and warranty)

“Thermostats in immersion heaters can be changed separately but this puts them at high risk and we do not recommend due to their constant use. Changing thermostat only, prolongs the inevitable.”

Is this bull?

Same here; I changed the thermostat then found that the actual heater was borked.

Yes it’s bull. Stats will last ages if correctly fitted – most early failures are due to poor wiring – loose connection etc.

As said above, change the stat on its own after isolating the power and removing the cap – scorching at the terminals will indicate loose connection.