Glow Worm 35 ci problem?

Our boiler seems to work when if feels inclined! This is not always when we feel like being warm! I have tried to eliminate obvious causes - there don't appear to be any leaks on the system or radiators, the outside escape pipe is dry. I have re-pressured the system from 0.7 to 1.0 bar and I could hear air escaping so I assume water was entering the system and it doesn't seem to be losing pressure regularly. It fires up everytime we run a hot water tap so I don't think it is the starting mechanism. When I check the thermostat it clicks to signify the current temperature but when we increase this sometimes it won't start and sometimes it will. I'm flummoxed! Has anyone any ideas?

Thanks.