Cheers Guys.......thanks for your inputs.
Gutter Airways - yes I did read that part of the FCOM - my understanding of that was that an ECAM will be generated if the altimeters disagree with the same pressure setting. Once the settings are made different - that particular ECAM goes away (because now the a/c logic expects there to be a difference) - to be replaced by another ECAM - which this time states that one of the guys in the flight-deck is a right "duffer" 'cos he has his QNH setting wrong.
That is the tolerence I am trying to find........and it would appear more and more likely that this info is not in the FCOM.
Much Appreciated anyway chaps ......
WM