The immersion heater on our HW cylinder is the old-fashioned sort, without an over-temperature safety cut-out. It's not practical to replace the immersion with a modern type, because the plumber who installed the HW plumbing, cleverly arranged it to pass straight over the top of the immersion heater housing, thus preventing removal of the heater without a significant re-plumbing operation. Added to that, removal of immersion heaters that have been in place for decades can have its problems, AIUI.
As a second best, I'm considering using a secondary external thermostat clamped to the outside of the tank near the top, wired in series with the immersion heater and set to say 90C. While I can find a thermostat that has an appropriate power rating for the immersion heater (Horstmann HCT2, e.g.), ideally I would like one that is resettable so that it stays switched off if the set temperature is exceeded, but I can't immediately find one. Does anyone know of such an item?