I am replacing the lead supply but the builders have installed 20mm pipe instead of the 25mm recommended by the water authority. The switch over has not yet been done and we have had installled a new combi boiler worcster bosch 40 cdi running on the exisitng lead supply. The mains pressure on the lead is not bad but with the combi it is lower. MY REAL CONCERN: as the builers have laid 20mm instead of 25mm will I loose pressure on my shower ( which hasnt yet been instaled.) Its bad becaue they have already buried the 20 metre pipe.