PulsaCoil 2000 Low hot water temperature - Unstable

My PulsaCoil 2000 runs funny recently. I sometimes have hot water, but the temperature drops a lot after a while. It is unpredictable. It sometimes works properly with steady hot water flow. I had the two immerser heater replaced last years. I checked that the storage water tank is hot, the heat pump is on, the indication light is Green and the heat exchanger is hot (except when the incoming water runs through it for a while). It seems that the system is operated normally. As the outside temperature is -2 to 2 degree at present, I wonder if this Pulsacoil system is not good enough to heat up the incoming cold water. Or maybe the heat exchanger is filled with scale. Can anyone advise ? If the heat exhanger is required to replace, can anyone advise an engineer to do the job in W2 London area. Thanks in advance

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Mike

We work in the W" area and could help with a site visit. But please confirm that when you had your elements replaced did they replace them for the correct thermostat rods? This I have seen to be a big problem and could be related.