WTF ... if you know its not fitted properly why are they still using it ?? he sounds like a right knob and hasn't a clue ,
so you have an engineer out from the fire manufacturer who says the flue is wrong , then you call the previous knobend engineer who you know fooked up seriously and let him reinstall the fire back to the way it was wrong . FFS !!! . who would you believe more , the original engineer or the fire manufacturer engineer ?????? . is it that you want this fire installed quickly as possible or is it the price of other engineers ?? something does not sound right or your not telling the whole story . report the ****Flavel ,However they sent in a guy who has told them the fire is not suitable for the flue