optima 901 C/H not igniting?

My optima central heating is not igniting, the hot water side of it works fine, when I turn the thermostat on the pump seems to switch on but nothing else happens, when the thermostat is turned off again the pump does not seem to stop until I turn the hot water on. I have removed the screw from the pump and able to turn the impeller ok, pressure is fine, any ideas?

paul
January 2016