Gas boiler flue and opening windows along with how far away from the house wall?

We have a new extension on the side of our house. The boiler flue runs up from the inside behind a false wall with a hatch to get to the bends for servicing. It also has a hatch in the roof to get to the other bend for servicing. When the flue exists the roof how far from the existing house should it be and how far... show more We have a new extension on the side of our house. The boiler flue runs up from the inside behind a false wall with a hatch to get to the bends for servicing. It also has a hatch in the roof to get to the other bend for servicing. When the flue exists the roof how far from the existing house should it be and how far from an opening window. 2 gas men tell us 2 different stories?
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