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?