I have just moved to a new property and have a wourcester 24i combi. When I moved in I had to have the pump replaced and the expansion vessel re-infalted. Since then it seems to lose about 1/2 a bar of pressure every couple of days even when the central heating is not on. I have checked under the floorboards for leaks but cannot find any also it doesn't seem to be leaking out of the overflow, HELP!