I took the extractor hood down to paint the wall and it looks the vent is going into the unused chimney, which I suspect isn’t good. I haven’t explored to see if there’s a flue going up the chimney but, with the top of the chimney being 3 floors up, I’m not keen to investigate. I suspect that’s too high even if there is a pipe. Any thoughts?
Not as bad as mine. The pipe went straight up and then stopped at the ceiling. No hole in the ceiling or pipe venting to outside. Just nothing 
I would have thought it would be vented outside somewhere, but yes 3 floors does sound excessive.
Some recirculate after passing through the filters but not all builders recognise this and still put the pipe on the exit 
Our cooker hoods both recirculate through filters. It works, but they do need changing at least annually.