pressure keeps falling?

I have a Remeha Avanta 28c ECO. I've returned from overseas after my wife had reduced the temperature to 20 deg C. I retored that and everything was fine for a day. Now it won't work properly. There are no error codes but the water pressure keeps falling every 30 minutes or so. I have gone through all the P settings and I think they are ok, but I don't know what the best settings should be for P4. I want central heating and domestic hot water and I want it on all the time, ie not controlled by the time switch. I am a pensioner and I am freezing.