Cheddyman, From wot part of the world do you come from?, What is the brand of HWS?, Is the leak from the back of the pipe or is the leak from the open end?
In Australia we have one HWS which is a Coil type HWS, the tank is a low pressure cylinder which has a element, this tank is filled with a combination valve, this valve is also the temperature & pressure releif valve, one entry of the valve feeds the coil. there is a vent in the top of the HWS to allow steam escape and a overflow from the tank from the pressure temp valve. both these overflow points are safety outlets & if blocked are dangerous.