Heatstore Shore - cold water

Wonder if anyone can give me some help?
Have a Heatstore Esprit II shower, fed from a storage tank.
Over the last week have experienced a slower flow of water and not much heat. Have checked all the knobs & wiring to see if theres anything loose. Just wondered if its anything to do with the heat exchanger. Looks simple enough to change?
Thanks in advance for any advice.

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