Colin, I'm pretty sure this is a new loom installed by Jigsaw when they fully restored the car for the PO. Then they added front and rear spots, USB, map light, Trip master, radio (is the radio wiring part of the loom?) etc etc. Those should be easy to identify and put to one side. I'm going to study the wiring diagram in Haynes but be assured I will be asking for help......!!
If not there I'm also going to apply some bulkhead dampening to make it a little less noisy due to the tubular exhaust manifold.
Iain