Thanks for that. I have turned temp up full, turned boiler off and on, put boiler onto timer and also constant. Tried to reset the boiler. No joy. Hot water is fine. The pump starts and pilot lights no problem.
Do you have a 3 port mid position valve in your system? If so you may find the actuator does not function correctly. Sometimes you cannot get CH to work alone, but it operates ok when there is demand for both CH and HW. The reason it works in this situation is because the boiler is powered from a different source, but as soon as HW is satisfied and the valve moves, the source of power is changed and if that source is faulty then you have no CH.
Power source for HW is from cylinder stat.
Power source for HW/CH is from cylinder stat
Power source for CH is from Mid position valve (orange wire)
If you have a 3 port valve this could explain why you think it seems to be working.