BG C1 Boiler (Bosch)?

Hi
I have a British Gas C1 boiler and over the last 6 years it's had 3 heat exchangers.
It is now showing signs again that it may need another one. The symptom is:
The pressure in the central heating circuit drops to 1 bar over 3-4 days when topped up to 2 bar. There are no visible leaks in the system, there is no water coming out of the safety vent but there is evidence of a recent leak from somewhere in the boiler.

The last engineer who chaged the h.e. said there was a fine hole through which water was passing as a very fine vapour when the boiler was heating.

If this is the fault ....... does anyone have any suggestions why it keeps happening?