Hi , it may be your heating motorised valve that is at fault . do you have a separate one for the heating and hot water?
No There is a three way valve fitted in the boiler. If it helps boiler is a Glow-worm 30cx
Oh , ok , its a Combi
what is the model of your thermostat?
Honeywell T48 (T6360B 1028) Analog thermostat.
Hi , it will be a fault with the motorised valve not motoring over and switching.replace the head or the whole valve