So, I'd alluded to the fact that I've been having loom problems on someone else's thread and I've finally sorted it out today.
The problem started when I was moving the wiring about in preparation for bonding the front bulkhead to the underside of the body. The bare metal capillary tube leading from the adjustable thermostat switch to the sensor in the top hose touched something live. There was a flash and a puff of smoke and a terminal looking hole appeared in the tube
Concerned that I might have also blown a fuse somewhere, I turned on the ignition and started checking that everything still worked. I then found that the entire circuit that supplies power to the indicators, dash lights, brake lights, heater motor, reversing light and windscreen wiper motor (which I was using to power the electric fan) had no power supply at all. I checked the fuse and found that to be fine, and then checked the input side of the fuse - nothing. The car still started and all the other circuits were fine, but there was just no power to one side of the fuse box.
Absolutely gutted, it was obvious that one of my repairs to the loom (that I'd made after cutting through it with my angle grinder) had failed. There was nothing for it but pull the loom out again and start testing the circuits until the duff one revealed itself.
After a day of checking all the electric spaghetti with my multi meter I couldn't find a single fault in any circuit, so I reassembled it all, minus a few redundant cables to make it all a bit neater. Battery reinstalled, ignition on and...
Still no power to any of the circuits on that side of the fuse box

I plipped the immobiliser and started the engine again just to make sure it would still start, then shut it down to stop it overheating and had another look through the wiring diagram to see if I'd missed anything.
It was then that I saw that the coil was also powered off the dead circuit. And as I was thinking 'how the bloody hell am I still able to start the engine if there's no power to the coil?' my immobiliser went 'plip plip' to let me know it had armed itself after its thirty second delay. My immobiliser! Of course! I'd accidentally wired the bloody thing in so it isolates the entire circuit that the coil is on when it's armed. I plipped it again and switched on the ignition and, hey presto, I have power to everything again, hooray!
Anyway, at least I was able to tidy up the wiring, which had got a bit out of hand, and I've also replaced the adjustable thermostatic fan switch with a fixed one, which is much simpler and neater -
I've gone for an 'on at 88 off at 79' switch and mounted it in the bottom hose. This now cuts the fan in at the right moment, just as the needle climbs out from between the two dots on the temperature gauge, has it back between the dots in thirty seconds and then cuts out. Perfect. I'd say that you'd need a hotter switch if you plan to put it in the top hose though, as the water coming out of the block will be hotter than the water coming out of the radiator.
Anyway, I've got a few days off now so I'm aiming to make some real progress.