T
tamz
Not a facetious comment (as if I would) but if the flue is correct in every other way, you could always secure the window shut.
Screwing windows shut is not recommended. Anyone takes them out or the window is changed at some point in the future and it will still be you left holding the bairn.
The risks are not worth the effort even if you fired a strip of nails round it with a nail gun.