Thanks for the help guys.
With your encouragement, I did some hunting and came up with the following.
I decided I didn't like the old loom arrangement of "down the drivers (Lefthand) side of the car, down behind the wheelarch, connectors to lights in the wheelarch, wires across the back crossmember into the area behind the RH wheelbox, more connectors and into the lights" arrangement, which is both difficult to install once the tank is in place and having connectors in such a wet, muddy and difficult to work on area behind the wheels has been a source of much trouble over the years... so we made up a new "split" loom out of 7 core cable ("trailer wire"?) and ran one cable down the drivers side, the other across behind the dash and down the passenger side. We had to drill a new 1" diameter hole in the horizontal supporting plate by the passenger door lock and another down through the RH wheelbox into the back of the lights to accommodate this. (Cone drill and grommets etc.).
The old (original) rear lights were a bit manky so I bought a pair of cheap surface mount square LED trailer light clusters from Truckstop for £8 each, which I thought were in keeping but more low profile and modern. Thw rear wings now have no provision for red repeaters on the sides of the rear wings (which I have checked is not an MOT issue...) so did not have to do anything with these.
This allowed us to keep the junctions inside the passenger compartment and only run solid wires down into the wheelarch to the lights. We also had a wire spare in the cables to act as an earth, terminated them it to a nice new earth connector under the dash and did away with the old "body earth" scenario that has similarly caused me problems over the years.
We put in a new flasher relay on a couple of flying leads back to the fusebox/block on the bulkhead to replace the old (round) flasher unit and took the indicator leads straight off the relay to our two "looms".
We had some fun working out how the brake lights were wired (ie. flashing them off when the flashers were working...) for about 40 minutes and decided to just take the feed direct from the brake light switch, split it to the two looms and wire them in complete. This took about ten minutes.
Sidelight were easy, just split the wire down the two looms.
Then discovered that we needed ballast resistors to stop the indicators flashing too fast (this LED stuff is all new to us...), £5 each from Truckstop. Wired them in parallel to our nice new connectors in the back rear corners of the passenger compartment - and we are done!
Thanks again for your help and encouragement.
John