did a cp12 today which had potterton profile 50e.checked to see if safety device worked,ugauge connected to burner pressure test nipple for burner pressure test which needed tweeking a little.turned off gas cock to boiler, burner went off as it should but no sparking at all. turned gas cock back on burner pressure stayed the same, so gas still passing through gas valve at burner pressure rate. had to isolate boiler gas supply. has this happened to any of you lot before.