Ferroli Optima 901 -occasional Hot water problem?

Hi I have a Ferroli Optima 901 combi boiler and have recently been experiencing occasional problems with no hot water.
This has been noticed when running the bath, with it initially running warm and then running freezing cold.

I think I may have loft gremlins as when i get the step ladders out to access the loft, go into the loft, look at the boiler to check the pilot light and system pressure and then try the hot water again it seems to of rectified its self!!

I have in the past (6 mths ago) had problems with the ch side loosing pressure which was resolved with replacing the PRV.
The pressure in the boiler was repressurised to 1.5 bar (while cold). and rises to about 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?

Is it possible that it is the air pressure switch or is that purely there to monitor the operation of the fan?

Sorry for the ignorance, trying to understand whats going on and hopefully fix it - if it is an issue thats caused by the heating being off!

Thanks Chris