Vokera Excell 80sp?

My boiler has been shaky for a while - it's been repaired several times in the last two years (think had the switch replaced and the pump)
For the last few months it hasn't ignited when I've turned the hot water on (the pump works but the gas doesn't flow past the pilot light.) I was able to ignite it manually by depressing and releasing the gas control knob but now that doesn't work. Does anyone have any ideas for what may be afflicting it and how best to repair it?
Many thanks