CH fine but no hot water

I have a 16 year old Boilermate A-Class BMA 210.

Recently, the hot water stopped working properly. I ran hot for a moment then cooled then up and down a bit. Warm enough for a shower but not hot enough for a bath.

CH working fine. Checked all pumps - CH, Boiler and DHW - all seem to be working and not slipping.

Suspected heat exchanger so replace that. No difference.

According to the display, DHW in is sitting around 43 DHW out is at 55 when no HW called for.

As soon as I open a tap, the DHW in number begins to drop and settles around 14 while DHW out goes down to around 34

Struggling to work out error codes - C1 to C8 all show 23 and all others C0 and C9 up show FF - can't find anything to tell me what error 23 means but am assuming that as C0 is FF, there is no current fault.

Is there anything obvious that I have missed?

The plumber has now replaced the pump servicing DHW along with the valves before the pump and after the heat exchanger.

Still no DHW supply.

The DHW pump runs constantly, the pipe leading into it is hot but the pipe coming out of the hear exchanger remains stone cold which suggests no water is being pumped through, so perhaps the pump is faulty.

However, what I don't understand is why the pump would run continuously - surely it only runs when there is demand for DHW? The question is, what causes the DHW pump to start?

I assume you have the manual for the unit if not this is the link http://www.gledhill-spares.net/documents/BMAClassOV-Iss8.pdf The hot water I assume is controlled by the sensor on the top of the plate heat exchanger in combination with the one on the return side of the heat exchanger (the setup looks similar to the older system like the Boilermate 2000). I assume the sensor detects the hot tap being opened by the temperature drop on the sensor (see items 7 and 8 on the parts list on page 6 of the manual). It would then monitor the temperature and run the the hot water pump (2 on diagram) at a varying speed to modulate the temperature to a safe level. The sensors seem to be doing something, I would expect the initial temperature to be higher, closer to the store temperature, on the old system up in the high 60's 70ish. What is the store temperature? One other thing to throw into the mix, I assume it has plenty of water in it (I know the top up pipe from the cistern can get blocked on other systems but the system usually gurgles a lot). These control boards if I remember correctly are used on different systems and they have a resistor on one of the connectors that tells it which system it is set for, there is an error code for open circuit for this resistor code 40 I believe. Hope these thoughts help please let us know how you get on.