Potterton Boiler fires up for few seconds then dies away. Had same problem last year, but it corrected itself - so loathe to get in engineer I can't afford to fix something minor. Gone through the firing up procedures but no joy. Last time it happened, there was work being done on pipes outside by gas board. They said there was no connection - could debris be in pipe? Could jets/etc bet intermittently blocked? Am strapped for cash please help