My bolier only fires for 10-15mins max then after this time, flame wont fire and i get no heating or hot water! - only way to get it to fire again is to turn switch to off, then flick back on again, works for 5-10 minutes, then flame wont ignite. Gets progressively worse. Divertor valve and circuit board replaced, but made zip difference apart from me being £400 out of pocket!! Corgi engineer has said it's a 'possible' problem with thermostat. Boiler is 6yrs old but engineer reckons it's in good order - yer right!!