Combi 24 cdi Pressure build when off !Help Please!?

Hi Folks hope you can shed dome light on this for me...

Combi 24 cdi Pressure build when off

Problem began with a leak
After consulting manual and stripping down boiler as req'd found a bad leak at the Gas to water Heat exchanger. Bought a set of O Rings (original parts)
lubed with silicone grease and refitted.
All was fine for a day or two and then it leaked again ,not too much but concerning.


Realised a pressure build was going from 1.5 to 3 bar overhnight ..... !Boiler Off!.

Tested Expansion Vessel for Air or water and got air
havent pressure tested it yet

Assume not possible to leak accross filling loop
as it a "Insert the T bar type"