There are stove installers that will lie to you about the need to line flues. One near us said "Sorry, can't touch that, mate, flue's not lined, but we'll do it for you," then quoted us nearly two grand for the privilege of letting them rip us off. The bloke who sweeps our chimneys is also a qualified HETAS engineer and is firmly of the "if it ain't broke, don't fix it" school. Ask around a lot!