Glow worm fuel saver F won't light!

I've just read another post which is similar to mine. When i switch on the gas valve clicks as the pilot lights, sparking as well. the light stays on and sometimes the main burner lights, sometimes only after 20 mins or so. since yesterday the boiler has started not to light at all with the pilot still lit. i have noticed that a second "click" seems to occur and whwn it doesn't i guess the main gas valve solenoid is not opening. I had the gas valve replaced just 6 weeks ago which made no difference. i have tried to adjust the sensitivity of the air switch, but again not luck! It's snowing outside and i'd love to be able to fix it ASAP!!! Thanks