Ok, I did a bit more testing, and on the 3rd item I mention in the last paragraph above, the device looks like this:
http://www.ebay.co.uk/itm/HOTPOINT-Dishwasher-DWT10P-DWT10P-2-IDL530UK-2-Thermostat-Thermal-Limiter-/361847530464
It has a brass cap and it is screwed onto a housing on the...