OK , here is what I found today.
Battery now fully charged and car can start as per normal yay
Voltage across battery terminals at idle 13.4V
Voltage across alternator shell and positive thick wire at idle 14.2V
Voltage across alternator shell and positive terminal at idle 13.4V
Voltage across alternator positive terminal and battery - terminal 14.2V
If I get a piece of thick wire and connect the battery - to the ground point the battery shows 13.4V
If I get a piece of thick wire and connect the battery - to any part of the engine block I get 13.9V
I beginning to think as suggested by other that it is a wiring problem and the grounding wire don't seems to be doing anything at all. I will undo the grounding point and give it a good clean and get another new one from the battery - to the block.
However, do I need to do anything to the positive terminal at the alternator? I heard later model of P38 have a positive wire from alternator to battery , is that true?
Many thanks and I am grateful for all your help , my P38 is now running again