Gas boilers question for gas engineers?

I have a gas boiler, which was found to be not sealed from the lug nuts at the top on the inside of the boiler. There was a layer of dust inside the boiler on the top covering. I have a strong smell of unburnt fumes coming into my living area. Can someone possibly suggest what if anything is going on with the boiler? I have done a test with a candle as I know that CO2 will not support combustion, but the candle stays lit. Any suggestions as to what my next step should be?