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 pressue with the CH off is 1.35 bar.

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?

Last time I posted this question someone was good enough to point me in the direction of the Diverter valve being the possible cause.
How do I locate the diverter valve and the associated micro switches? Anyone got any other ideas?
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!