I have mentioned this subject in other online areas, but in case anybody has the same problem that I had. The clutch was slipping on my 1971 OIF A65 Thunderbolt. Whilst repairing the clutch, I found the oil seal split on the alternator wiring. Stupidly I pulled the wiring out without tying a length of string to it. When refitting the wiring, with new seal and pulling through to reconnect, I found the wiring was 3 1/2" too short. I had pulled the wiring UNDER the gearbox casing and up the rear of the engine - too short. Head scratching time, baffled, couldn't figure it. I then had another look at the parts list, and could see, there is a gap over the TOP of the gearbox. Pulled the wiring through again up and OVER the gearbox - BINGO - the wiring is now long enough to reconnect. Relax, pint of beer time.
I have one of those, will bear it in mind for the future.