FAULT WITH DOMINA 80E?

I HAVE A DOMINA 80 E WHICH HAS THE FOLLOWING FAULT:

I CAN ONLY GET HOT WATER WHEN THE CENTRAL HEATING IS FIRED UP AND RUNNING !!!

ALSO CENTRAL HEATING WILL STOP AND SHUT DOWN LIGHT WILL BE ILLUMINATED WHEN UP TO OPERATING TEMPERATURE. RESET BUTTON WILL NEED TO BE SET TO RESUME BOILER OPERATION.
THERE IS NO ROOM THERMOSTAT FITTED.

PLEASE HAS ANYONE ANY SUGGESTIONS TO THIS PARTICULAR SYMPTOM