Update on progress - My new loom turned up promptly from Redline (£240+£12 postage) and I have taken the advice offered:-
1. Rivets drilled out, including the 2 big ones at the sides and the 4 nuts, scuttle now removed. M4 Rivnuts and machine screws ordered along with a Rivnut insertion tool (£9).
2. Have taken plenty of phots of all the important places for future ref.
3. Old loom now out but still connected where possible, will fit new loom and once lined up will then remove one at a time of the old connections and replace with new.
Question: I have snipped the smaller branch loom going to the rear just in side the gear stick housing, ensuring I have plenty of length to work with. All these wires look in good condition. Should I snip the new loom and join to the good condition rear old loom with a suitable connector? Or carry on all the way with the new loom to the rear?
Guys thanks for your help and advice, most graciously received - thank you all.
Steve