Riello 40 Burner problem?

I have a Riello 40 G7 140-169 Code 3745959 Type 459T55 kerosene burner which following an oil fill has started to lockout. It happens either on starting or after reaching the thermostat setting when it goes through the purge cycle and fails to ignite.
The fuel and mainline filters are clear and fuel not water is reaching the pump. There is a very good spark, no sooting and no excess fuel in the boiler. The nozzle has been replaced, the magic eye is clean.
Once the boiler has reached thermostat temp it turns off and locks out on restarting, pushing the red reset button initiates purge but results in lockout again. At this point if the fan is rotated even slightly the burner will then fire up OK on pushing the reset button. Any ideas what may be the problem?

Brian Jones
February 2011