Get the expansion vessel checked in the boiler, its most probably flat and overpressurising your central heating system causing leaks.
The plumber (I keep calling him a plumber, but he's a Gas Safe installer) seemed satisfied that the expansion vessel was fine - after he replaced the PRV for one which didn't leak, we pressurized the system and when the boiler was running it rose to about 2.5 bar. He said that if the expansion vessel was flat then it would go higher than that and water would start coming out of the PRV.
Oh dear! Going up to 2.5 bar means nothing without knowing what it was when cold.
I suspect that is the problem.
Set to 1.5 cold and tell us what it is when hot!