The gas burner on my vokera mynute boiler fails to ignite. The circulating pump and flue fan are still operating.I have taken off the cover and cant see a spark or hear the clicking i would associate with an ignition unit and there seems to be no gas flow either. Any advice on how i can identify the offending part would be appeciated. It might be worth mentioning that the boiler also loses pressure over a day or so but has alwways responded to topping up.