Having replaced the burnt out wiring, I noted that the alternator was not charging the battery. Took this off the car and found that the regulator was faulty. I sent off for a new regulator – less than a tenner, and easy to fit. I checked the diodes, and the coils which were all ok. With the alternator re-fitted, charging was restored.
I have re-routed the main red wire from the alternator round the engine, and connected it directly to the starter motor, and therefore the battery. The other connection on the starter motor is to the pre-heater relay, and this was replaced with a direct connection outside the wiring loom. The existing connection to the pre-heater relay was traced through the wiring loom, and will be used as the main feed to the rest of the system. In the UMM, this connection is unfused up to the light switch, ignition switch and possibly the clock. This is slightly risky, in that if you have a short circuit on the lights, or any other of these circuits, you are likely to burn the wiring out or start a fire.
In order to make this safer, I have cut the main feed and inserted a maxi-fuse. These are available in 20 amps, 30 amps and more. Which should be enough to cover all the electrical systems fed by this wire. This is everything but the starter, alternator, and pre-heater relay. I mounted it near the original fuse box, under the glove box.
Here is a picture of the maxi fuse, next to a regular sized one. I shall continue to tidy up the rest of the wiring as I go along.