Have an intermittent problem on a Sime 100 boiler?

Have an intermittent fault on a Sime100 boiler. Sometimes works fine other times does not work at all for hot water of CH. When not working for either HW or CH or both demands I can see the spark attempting to light the pilot but no flame appears. Other times works fine. Any suggestions?
Should the pilot burner be working all the time or not?

PAC
June 2012