That's exactly why they came out because I reported seeing a yellow flame, he checked the gas tightness at the meter and then checked for gas and carbon monoxide traces in the house which were none, he then got me to turn on the boiler to see if the flame was yellow, which it wasn't he then said that there was no immediate risk but said not to use the boiler into a gsr engineer comes and looks at it, which British gas are doing tomorrow. I have lived in this house for nearly six years and have had bg out a few times, once for a leaky radiator and twice for expansion vessel related issues! Basically the bottom line is the boiler needs fixing and bg have gone a bit ott with insisting national grid gas come out first, due to this yellow flame I saw yesterday but not since!