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.

Vic
March 2009