Baxi combi boiler losing pressure?

Having carefully read your description/question I would be inclined to go along with the likelyhood of the expansion vessel being faulty. The simple way of checking this is to check fill pressure is 1 bar and fire boiler up on heating and watch the pressure gauge. If it rises rapidly to over 2-2.5 bar then its most likely the vessel either needs pumping up or replacing. If you are going to check its pressure via the schraeder valve then you need to depressurise the boiler/system first.