I'm not the OP.
I fitted my stove about 3 years ago. Vitreous enamel straight into 300mm clay flue.
Never liked the idea of putting metal into the chimney. The MI's said fit one, I think hetas say fit them too and I think BC require it also. At least in NI afaik.
Been working fine and performs well.
Most official bodies are towing that line afaik
No cracks on the breasts yet to allow leakage and if any appear I can always wallpaper over them.