First of all you have to make sure the fire you have is compatible to the flue. There are different classes of flue and different classes of fire so you could not install a class one fire into a class two flue. Also it sounds as if you have a flue liner installed and these cannot be extended or modified in any way, so it would need to be replaced if it does not fit straight onto the spigot of the draft diverter of the fire. And you're installer will know all this and advise you anyhow. If you are in the UK you will need a CORGI gasman to do the job by law and for you're own safety and that of you're family and neighbours. Also in the UK all new or replacemant gas installations have to be notified to the HSE. And certificated and Only a CORGI man can certify the work for you're home insurance purposes and to satisfy the home owners informatin package.