Ariston ACO 32 MFFI - Losing Pressure?

Hi all, I have got the above boiler (out of warranty) which when pressurised (Via Filling Loop) is losing full pressure in about 2 hours. When pressurised I get heating, and DHW (when DHW is on boiler seems to stop central heating, don't know if this is normal or not) I have checked all the radiators and can't seem to find a leak, I have used fernox internal leak sealer in case it was a copper pipe leaking which I could not see. I have checked the external pipe connected to the PRV and that is not passing water (so seems to be ok) could this be down to the Pressure Vessel or Heat Exchanger? and if so how would I go about checking them? or could it be something else? any suggestions/help very much appreciated.