Yes! I had alot of trouble with this happening after I replaced our immersion heater last winter.

In trying to solve this turned down the thermostat to the min setting - it still randomly tripped.

I replaced the thermostat with a (identical) new unit - it still randomly tripped.

I replaced the entire immersion heater again - it still randomly tripped with both thermostats.

During this debugging process I modified the outlet on the hot water cylinder to allow me to insert a 300mm K-type thermocouple into the water. By logging this thermocouple I was able to prove that the water temperature never reached anyway even close to the 90C cout-out point of the over temperature trip - yet it still tripped!

Just when I was reaching the point of despair - it stopped tripping & has behaved itself since the spring (no, this doesn't coincide with the central heating being turned off!). The logging is still in place & have have noticed that the thermostat switch off point is slowly creeping upwards. We are in hard water area & i'm wondering if this is happening as things are slowly being covered in limescale.

where in the water is the thermostat measuring,  and orientation of this relative to heater element ? just wondering if it matters if the heater and stat are one above the other in terms of convection - it may not be stirring itself as well as you think.

I did think about that... I tried rotating the the immersion such that the element was above the 'stat (this is a immersion mount on the side of the dome of the cylinder so it projects through the water at an angle) as well as above it - it didnt make any difference.

I did think about that... I tried rotating the the immersion such that the element was above the 'stat (this is a immersion mount on the side of the dome of the cylinder so it projects through the water at an angle) as well as above it - it didnt make any difference.

So, this evening, the water boiled!

I just happened to be in the right place at the right time - I heard the boiling start & the first of the steam bubbles come up the vent pipe! I quickly switched the immersion off so no real damage done.

The tank is way to hot to touch at the moment, the logging thermocouple is still showing 97C, but tomorrow I will have a closer look at the immersion heater thermostat. It will be interesting to see if the secondary over temperature cut-out has operated - I suspect not.