OK I finally tracked this down and sorted it. Running a single jump lead between the Battery -ve terminal and the alternator body proved it to be an earthing problem - the fault immediately went away - so now it was a case of tracking it down. Having cleaned and re-fitted the earthing straps I could get to easily I then took the Battery and Battery tray out to get at the engine earthing strap. (Cunningly hidden final securing bolt, BTW.) This revealed the engine earthing strap to be loose and t