You need to take it up with building control, they have the final say. No point whinging on here about it as we haven't got any power to change it, and personally couldn't give a toss if you walk through a steam cloud or not.
Sorry if you take offence but reading through the thread it's obvious you have a distinct disliking to this extension as a whole and your grasping onto this flue issue like its going to ruin your life. Sometimes things change and someone else's benefits/actions can have slightly negative effect on someone else. My advise is get over it, its a bit of steam not an incinerator outlet.