Hey RobRW
Thanks for the info,after further trouble shooting we found the RH Engine Pre-cooler to have a 3 inch crack which is leaking a serious amount of P3.
The snag has actually progressed a bit,the Cabin alt high annunciator comes on in the cruise with the rate still hunting between 500 foot climb and 1000 foot descending.Only once the Enviro is placed on MAN COOL and more heat is introducd into the cabin(bypassing the ACM) does the Max diff and cabin alt high sort its self out.The Pre-Cooler will be replaced within the next few days so will update the thread if the snag clears.
Cheers Gents,thanks again