Any leak will cause the pressure to drop - I hope that is the only one you need to deal with!Here is the boiler pipe work. I have a magna clean fitted and have now notice a leak coming from that which I think may be causing the problem. I'll see if I can find a way of taking it off to empty it out and refit it. Do you think that could be causing the problem?
However I would expect that the system would re-pressurise in the normal way. I was expecting to see a black levered valve both ends of that short bent copper pipe near the gauge. As those would be the one to open and connect the central heating to the cold water mains for re-pressurising.