There are numerous Youtube videos showing how to reset the safety secondary cut-off on immersion heater 'stats. The problem must be a common one. 

https://www.youtube.com/watch?v=td3SuGeTefM

Z.

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.

Its a long while since i replaced an immersion heater stat. However i do remember when the safety high limit inbuilt stat thingy came in it was a right old pain in tge neck with the high limit stats tripping on non overheated circuits. I concluded that perhaps the setting of them was a bit too sensitive. Im talking top fed 18 inch rod stats here