If a boiler is leaking then the water should be pooling underneath so check that first. Water can be leaking out the pressure release valve. These can sometimes leak and water will come out of a small pipe that runs from the boiler outside.
See if there is any water coming out of this. Small drops can cause the pressure to drop.
Maybe you have a leak then there are some leak sealers which are sold by Fernox but I don't know how effective they are.
Well if you have to get a plumber could you get the guy that installed the system as he would know the likely spots for leaks, at certain joins etc,.