I would have thought the car would adjust fuelling from cold air map to hot but cannot see it being directly related to pressure in the water system as nothing is reading water pressure.

What management are you on?

I think you are going to need to replace pressure can and see if that solves the pressure change, then rule out head gasket issues etc.

Where are you based? I suspect someone like Mark Shead or MSD could diagnose and fix quite easily.