Ferroli Optima 901 -Intermittent Hot water problem?

Hi I have a Ferroli Optima 901 combi boiler and have recently been experiencing intermittent problems with no hot water. Shortly after this started I lost all pressure in the CH side which turned out to be a faulty pressure relief valve. This was replaced and the system repressurised to 1.5 bar (while cold).

I noticed that this pressure rises to 2.25bar when the CH is on, i assume that this is correct due to the expansion caused by the increased temperature.

The only thing I can think of is that the problem seems to happen when the CH is not on (or at least thats when ive noticed it) thus the CH system pressure is at its lowest. I know the Boiler shuts down at 0.5 bar, is it possible for this sensor/valve (or whatever) to be faulty? Or any other ideas?

The guy that replaced the pressure releif valve (wish i had changed it myself now!)mentioned something about an expansion vessel!? Whats one of those? Sorry for the ignorance, trying to understand whats going on and hopefully fix it before summer - if it is an issue thats caused by the heating being off!

Thanks Chris