thelia 23, won't fire up (pilot lit, just won't heat water)?

When I try to turn the heating or hot water on, the fan keeps clicking between fast and slow, approx every second. When I check the voltage to the solenoid which controls the gas valve, it's nil when the fan is on slow and approx 4.5V when fan on high, not enough to open the valve! I have replaced the PCB and the fault is still the same. When I take the front off the boiler, fan is at high speed, pump is running and I have 4.5 V at the solenoid. I just can't figure this out!!