Got a Carver II blown air heating system in my VW Compass Calypso van , tried to use it recently and can,t get the gasfire/heater to stay alight ,followed the faultfinder in my handbook i.e. new gas cylinder ,hold down button to bleed gas through etc , its igniting off the button (not everytime)and stays alight on the preheat setting but as soon as I turn the dial to full setting it goes out ?