I checked and double checked the diff assembly and all was fine. I tightened the diff and then set the screw back a little to allow the cup to rotate. I have now fitted the diff into the rear housing and checked again and all seems well!!
I don't know exactly waht was going on before but whatever it was, it aint doing it now.
I have the front diff here and it is loosening/tightening itself when I twist one of the diff cup one way or the other!! All is as it should be but I am mystified!