Did you know that you have just connected the gravity fed hot water system to the incoming mains drinking water supply, by fitting this mixer/shower bath tap??? This is illegal and it needs building regulations approval and should only be done by a qualified plumber who would have taken a balanced cold supply from the header tank and then you would have had equal pressure with both hot and cold taps then you could have pumped both if required. As it is now you risk polluting the drinking water. risk been heavily fined and it will have to be re-done properly as described.