Pressure reducing on my Bosch Worcester 24 CDi Combi Boiler?

A few months ago i had problems on my boiler with 'rising' pressure. This turned out to be the Heat Exchanger at fault letting mains cold water feed directly into the expansion vessel. The heat exchanger was replaced and after a few very frustrating months my boiler was working as intended again. This was replaced in December but i now unbelievably have 'reducing' pressure. The PRV does not appear to be letting by at all. There is still air present in the vessel but i can't be sure this is at the correct pressure so it may need recharging but this was done just two months ago. I have no physically visable leaks on the floor below. Over the last few days, I have therefore had to top-up the vessel via the filling loop to maintain running hw/ch.

Q!. I know there is a diaphragm on the vessel that may of possibly perished but i'm not sure where this leak would feed out to ?

Q2. Does anyone know of any other leak defect possible on the expansion vessel.

This has been a long and cold enough Winter without all these heating problems.
Any help would be greatly appreciated.