how can i fire up my netaheat boiler?

just got back from holiday,and the boiler won't fire up.I'm getting no spark,and no noise to indicate that its trying to spark.The fan is running and all of the external valves seem ok.I've bled the gas and am wondering if its something to do with the gap between the flamesheild and the electrode.If anyone knows what the gap should be please could you let me know before winter sets in....thanks