Thanks guys! If you recall, I was about to rip out that flap behind the rear wheel to check the wiring on my 98 D90 to try to cure confusion with the indicator/brake light blinking. I found the break in the earth wire just as described. Part of the loom had come adrift and had been rubbing on the transmission shaft. I connected the two broken black wires and that has cured the problem.
BUT I now have another! The fuel gauge (only) is dead! Everything else seems to be working OK and the fuses are in tact. I assume that is something to do with one of the other wires in that location. Can't remember the colours but two other wires run parallel to the two earth wires and one may be broken inside the insulation. Moral: Make sure everything works before wrapping the wires with insulation tape and tying neatly out of the way with cable ties.
I bet it rains too! :(
Derry