Having arisen from my medicinal stupor - what day is it? Thought I'd better post an update on the loom.
Decided to see how bad the damage really was. I cut the loom about a metre past the point where it goes under the car so that I could join it again, if I had to, if the loom could be repaired. That way, the repair would be invisible from above too

Took the rear portion of the loom out of the car and onto the bench. Cue cup of tea. Stripped back the braiding to reveal wires and get a closer look at the damage. Basically, the wiring up to the point where the fuel gauge circuit goes of to the tank sender unit is fine. All the wires in the loom are separated and free with no sign of fusing/melting together. Wires at the point of cutting are also fine so this time I managed to stop the whole loom being affected by the short. Past this point, where the loom splits to go to the rear lights and tail light, the wiring is basically toast.
Having discovered this, I'm pretty sure I can now effect a complete repair without having to buy a new loom

Relief all round - cue another cup of tea. Never really bought wire before so what spec should I look for? Will need green(white), green(red), green(purple), red and black.
At the same time I tested the switch and, as far as I can tell, it works as it should. No definitive evidence of a short circuit on the power side but will check more thoroughly once I have repaired the loom and fitted the necessary in-line fuses to protect the rest of the circuit.
I feel the evening cold symptoms coming on again so will now finish off the bottle of Islay Lagavulin 16 year old that a good friend bought me a couple of Christmases ago. See you in March
