Can you trace the 15mm copper pipe that exits the wall about 3ft below the flue and put a plastic bag around it
refill the system and put it through a heating cycle and see if the bag catches any water
the bag did catch a little water,but now the pressure gauge fluctuates from less than one bar,and shots up to 2 bar?
Ok, sounds like you need a new PRV to stop it dripping at less than 3bar, also there may be a problem with your pressure vessel - this is why the pressure is fluctuating when heating. This part of the boiler should cushion the boiler against the expanding water.
It may just need the balloon inside reinflating