the covering on the donor (Lada!) column wires has melted a little (previously). I guess this means that equipment was drawing too much current....or could it be a switch or fuse box problem? Any tips for trouble shooting wlecome!

Tim
ps obviously intend to cut out sections and replace.,,

All of what you mention could be the culprit, but Lada's never used the best quality in their construction, and corroded wires were common, the resistance increased, and so did the heat, If the insulation is just heat fused together, and not black and charred, this is probably the reason

The Black/Yellow ignition wire on our 2.9i 4x4 donor had a similar problem. The insulation wire was obviously overheating and had; in some places compromised itself. The bad area's are chopped now, but I had it on good authority that the wiring on the top models wasn't quite up to scratch regarding suitability - allegedly!

ideally it shouldnt be that equipment was demanding too much current for the cable because the fuse should be rated low than the cable current carrying capacity.
could it be that an external heat has caused it ( can't think of any myself).
sounds more like an individual fault to that car, has somebody changed the correct fuse for a higher rated one and added an assessory like spot lights/fog lights.