How can I fix my Halstead Finest Boiler?

I am still having a problem with my Halstead Finest Boiler. The Heating works ok by itself, but when a tap is turned on for hot water and the heating is off, most times the boiler goes to lockout with a continuous red light. Once the electric is switched off and on again it fires up but soon goes back to lockout. If the boiler is on for heating and a tap is turned on, it sometimes works sometimes not. Also when trying to run a bath, the boiler keeps shutting down and firing up again and eventually goes to lockout. It seems the problem is with the demand for hot water. Could it be the flow switch or the thermistor not working correctly or could there be a problem with the pcb. The Heating side appears to be working ok by itself. Any ideas???