It is difficult to diagnose remotely, however it sounds like a circulating pump failure. The pump maybe spinning but the impellor may be defective. Also is the gas boiler an open vented model?
I think I've identified the source of the problem.
The temperature on the pump stat was set to 65c.
I reduced this to 30c and the pump kicked in sooner.
What would be optimal temperature for this stat?
If you turn the stat down to 30C it will heat the hw cylinder to 60 - 70C or whatever it can achieve, once the fire is goes out and cools the circulating pump will not switch off until it dumps the heat from your hw cylinder and reaches back to 30C.
It sounds like the thermostat is too far from the stove. The thermostat should be located as close as possible to the flow pipe on the stove. It is located in your hot press, then it is too far and will require re-locating.
You originally stated that the pump was spinning therefore how is now suddenly working?
If you turn down the stat and the pump switches off then this is correct. If you leave it set to 60C, the pump will switch on when the flow pipe temperature reaches 60C and switch off when it falls below this. If the pump is set to a low temperature the pump will operate at low temperature and eventually drain the heat from the hw cylinder coil and dump it in the stove back boiler. You could slightly lower the temperature of the stat to say 50 or 55C but you should heat the water to at least 60C regularly to eliminate the risk of legionnaires disease which will not survive at temps above 55C.
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