recently changed 3 way valve as had no hot water. this corrected problem. after returning from a weeks holiday the water & heating won`t work. When I turn the hot tap on or heating on, the boiler is clicking as if its trying to egnite, if i remove front panel and loosen screws air gets behind steel plate and flame egnites, returning water and heating. obviously this is not how the boiler should run, any ideas ????