Thanks for your patience. I finally got the time to throw myself on the kitchen floor.
Crikey, access is crazy on this thing. I had to take the right-side panel off, and could just get a screwdriver in to prise off one terminal from the heater. Then measured it and it was 24 Ohms, across the two big terminals that stick out of the white bit. Does this mean the heater's OK?
I also took out the Power Module PCB, as that was in front of me. I've attached some photos of one of the relays. Would you agree that the relay has got very hot and cooked its own solder joint?
The white connector below it in the photo is where the thick wires connect, so I'm guessing that it's very likely this is for the heater.
This PCB is about £140, so I'm going to try replacing the relay itself. I could possibly make it work by just resoldering the joint, but I want it to last. I'll see if I can get a higher-rated one that hopefully won't be getting as hot.
I'd be grateful if you could confirm that the heater resistance sounds right, and that I'm heading in a sensible direction.
Thank you very much.