Thanks, Mia. Your answer to my question reveals the depths of my ignorance!
Found the thermostat hidden under a black box on the side of the heater. Checked that there is mains power reaching the thermostat. No clicking, however, when I turn it from 1 to 5 (it was on 4.)
So looks like the thermostat is shot?
Ray, tried to bridge the connections, but my sausage fingers found the space too limited to allow me to work. My appreciation of the DDZ engineers has increased still further!
Lander: thank you. My boiler is a QL, and I found the manual here:
http://great-water.com/store/PDF/files/manuals/Install_WaterHeat_Eng.pdf However, the instructions are less than clear about the cut-off: "Our thermostats are equipped with a high temperature cut out safety switch, which is an electro-mechanical device that opens the current circuit and disconnects the electric heating element if the thermostat fails. When activated, the pin located
near the temperature regulator dial is released. To check that the pin has not been released and is set correctly, check that the pin is lying a few millimeters below the structures edge."
HOWEVER, having plugged, unplugged, prodded and circuit tested, we now have hot water! I have no idea how this happened - but wouldn't have had the courage to try without all your suggestions.
Question - attached to the cold water inlet to the heater is a red tap
with a turn arrow. However, rather than lefty-loosey / righty-tighty turning, it seems to turn in 1/4 turn increments, and one setting engages the water pump continuously, the other doesn't. I wonder if this is how it's supposed to work?
Thanks again for the assist. Looks like the rain has stopped too - good news!