I believe that the Flue has to be sealed to the stove itself , this can really only be achieved by putting in a flue . It doesn't take that long to do it , trouble will be the cost as scaffolding would be required to access the chimney outlet . I also think you have to fill the void between the chimney and the newly installed flue with XXXXXXX , can't remember the name of the stuff !
Vermiculite is the stuff.
Most chimnies will have an original clay lining which lasts for decades if used constantly-alternativly,it will fall to bits if the chimney hasn't been used in a while.
A new steel liner is always a good idea as these new stoves are designed to burn as efficiently as possible and the heat is dispersed out wards instead of up the chimney.
You should not need to fill as these are usually sold in a size to fit snugly inside the chimney but if the new stove is to be set into the void where the old fireplace was and is not freestanding,then use vermiculite to fill the void behind and around the stove.
