Update on my washing machine situation:
I took a chance and bought a replacement pressure switch for it, but that didn't make any difference. I checked the control module PCB for damage and didn't spot anything, tested the thermistors for the dryer and water heater and found them to be ok, checked each of the water inlet valves, the drain pump, drain hose and pressure switch air-hose without finding any problems. I took the program-select rotary dial mechanism apart, cleaned the electrical contacts in it and reassembled it, to no avail.
I then did some more research online and found a post on a forum where someone had found a burned-out trace on their control module PCB and had fixed their machine by repairing the trace, so I took another look my own control module and found a small scorch mark that I'd missed earlier, where a thin trace had burned out. I soldered on a replacement jumper wire, reconnected the PCB to the washing machine, turned it on and was rewarded by the familiar and reassuring sounds of it filling with water and starting the wash cycle as it's supposed to.
A few seconds later, I heard the less welcome sounds of it gurgling to a halt and signaling an error. Now it's flashing it's LED 7 times, which is progress of a sort. Seems that the relay for the heater might be stuck.
bet it is as simple as a broken spring.
It was a broken wire inside one of the connector tabs on the water heating element, which I think jammed the relay and burned out the trace on the PCB when it went. I've repaired the connector on the heating element with a screw terminal that I stripped the nylon sheath from, some solder and some silicone sealant, and the machine has now made it through a full wash cycle without any problems.
I'm quite pleased that I've saved a £250 tumble dryer and a £450 washer-dryer from being scrapped and replaced.
phobia.