I have a Mira Excel gravity fed mixer shower which is fed from the tank in the loft and hot water cylinder. The hot water pressure is fine but the cold is very low. The tank is about 15 feet above the shower. There is a gate valve in the supply line which doesn't seem to fully shut off. Is it possible / comon that this valve could have broken up inside and is restricting the flow as it is not opening properly? All other pressures elsewhere in the house are fine.