Glow worm 35 ci combi dripping ?

Hi i have recently replaced a broken bleed valve on a radiator which caused the pressure in the boiler to reduce to 0.7. I re-pressurised the boiler to 1bar using the blue tap underneath the boiler. As soon as i closed the filling tap a few drops of water leaked from the bottom of the boiler. Having read around and looked underneath the boiler it looks as though the water was coming from the reduced pressure zone valve. The boiler has been working fine with both the heating and hot water coming on, however we did notice that the pressure dropped to 0.5 bar last night once the heating had gone off which caused a red flashing light on the display. Once again, i re-pressurised the system and water fell out of the same valve, however it seems to be working fine again. Why would this be happening? I assume it is indicating something is faulty - would anyone know how much it would be to replace the parts and get it fixed.