Hi , there are a couple of things it may be
what are they
Hi , firstly its important to find if the fault is with external controls or within the boiler
Do you have a programmable room stat or programmer and separate room stat . and if so is it wireless?
Ok , need a little help here , so how do you turn the hearing on?
heating is on a timer with no room stat.
Ok , the timer could be at fault .but if there is power coming from it to the board it could be either the flow and return thermistors ( easy and cheap to replace ) or the diverter valve/diverter valve head. Take off the diverter( three way valve ) , circlip holds it on . when the heating is switched on ,make sure the motor is moving / turning. The valve has to motor the valve across for hot water. If there is no movement , check for power to it.if there is replace the valve motor head. Also with the head off check that the valve itself moves. If this is ok , then replace the thermistors , flow and return