Tried putting a jumper cable between the engine block and battery negative terminal? Makes a nice chunky ground path.
I don't know if I said it earlier but check the alternator is earthing to the engine correctly too. I had that once years ago & took me ages to find.
Did all this back in March when the problem first appeared, cleaning up and adding an extra earthing strap between body and engine block did fix the problem. My jump leads are on loan at the moment to my brother, who quite conveniently lives 80 miles away. I'm going to try adding another strap direct from alternator exterior to a nearby grounding point, along with a chunky cable between B+ and battery +ve.
Only things on that main positive feed line are alternator, starter, EMU and battery. Going to disconnect each one in turn and see if any resistance along the cable changes. If this makes sod all difference then it's pull out each of the many millions of fuses isolating a circuit at a time to see if there is a change in voltage.
Don't have any more car tinker time this evening as have guests for dinner who are due any minute and I'm still yet to get showered.