I have POTTERTON PERFORMA 24 COMBINATION BOILER WHICH WAS FITTED 3 YEARS AGO. RECENTLY IVE NOTICED THE WATER PRESSURE NEEDLE START TO RISE. NORMALLY THE BOILER RUNS AT ABOUT 1.5 BAR BUT NOW CREEPS UP TO 2.5 AND BEYOND. I HAVE BEEN DRAINING SOME WATER OUT OF THE SYSTEM IN ORDER TO LOWER THE...