Thanks for all the responses. Looks like a flue liner for both but I might end up with a wood burner instead of the gas fire so at least some gain. My first time on here and a very positive experience so thanks again.
Thanks for all the replies and the safety advice. Guess I knew I needed one flue liner and now possibly two. I made a mistake saying it was an lfe. It is open flame with imitation coals lodged on top. Is it worth getting a camera survey first to see if there's an easier solution. Thanks again.
I've a small open fireplace in the cellar which has been NACS swept with a Colt cowl fitted but smoke exits from the living flame gas fireplace in the room above even though it has a separate flue. Any ideas or is it definitely a flue liner I need. I have a Co2 detector.