Thanks - I was using Thermocouple as a general term, but I've now learnt that I meant RTD. For control I'm looking at a Hotwire thermostat (normally used for heated flooring) with a contactor for the load and a 10K RTD sensor. I have IOT stuff installed around the place but want higher reliability for hot water.
Good to see a similar approach has been broadly accurate for others.
I think a pocket/thermowell would be first choice if I have a spare threaded nipple on the cylinder, if not I'll play with putting it by the element or under the outer casing.