Pulsacoil Stainless lukewarm water

Hi.
If I turn a tap on full, after the initial burst of very hot water the temperature only comes up to lukewarm.
I have renewed the plate heat exchanger to no noticeable effect. The water coming from the thermal store is roasting Hot, you cannot hold the pipe. The water returning to the thermal store is cold so all the heat is being transferred within the heat exchanger. If I reduce the flow then the water does indeed heat up.
So before I start to change out other items does anyone have any hints.

Further to the above.
I have measured the flow through the bath tap and it is approx 8 ltr/min which seems correct to me.
The system is about 5 years old now and this issue has come on gradually recently.
Pulsacoil PCS180SCAL

Hi the fact that the pipe coming out of the heat exchanger is very cold seems to point to a lack of circulating hot water going into the heat exchanger which might be an issue with the PHE pump or the pump control pcb or the two sensors. If this is the correct manual it has some information on fault finding a lot of which is engineer level stuff but there is some information on what the LED on the pcb mean in respect of faults etc. https://www.manualslib.com/manual/994191/Gledhill-Pulsacoil-Stainless.html?page=23#manual

At great expense I changed both the plate heat exchanger and the pump. No difference. Then did the sensible thing and contacted Gledhill Technical help. They talked me through various checks. Eventually diagnosed a blocked check valve in the pump outlet isolating valve. This had become scaled up and stuck. I just pulled the check valve out. Hot water now in abundance.

PJW wrote:At great expense I changed both the plate heat exchanger and the pump. No difference. Then did the sensible thing and contacted Gledhill Technical help. They talked me through various checks. Eventually diagnosed a blocked check valve in the pump outlet isolating valve. This had become scaled up and stuck. I just pulled the check valve out. Hot water now in abundance.

That's an interesting one. I've not encountered this particular fault on a Pulsacoil Stainless yet. Thanks for posting the solution.