Had three quotes(all hetas reg) for stove installation, 2 guys say liner, one guy says no need and that anyone who quotes for one is just taking the mick and after easy £’s! Who speaks the truth? Or how do I know for sure? Its a gift so I’m not shelling out personally but obviously the stove gift will be cheaper without a liner. Guess I could contact hetas.

okay, you need a proper chimeny and builders opening, the builders opening will be say 560 x 400 ish and about 150 or more deep, if the stove has a rear flue exit you ll need about 130-150 from the end of the flue to the back of the builders opening.
the flue itself should be your standard 9” brick flue with a nice terminal on top. it should be complete no bits filled in or bricks missing ( stick your head up with a torch.).it should take a ‘smoke bomb easily (you can heat the flue for 10 minutes in extreme cases before lighting the smoke bomb, if all the smioke is carried out of the terminal bobs your uncle if not splash on a sweep ( would reccomend any way)
liners cost circa 180 for a 5 inch liner inc terminal they can take 15 minuites or a **** week pay by the hour, two man job one on roof one below

Oh! What if I’ve opened up the fireplace and have a stove with a top flue exit sitting in it below the chimney?

it does depend on the chimney to an extent. my father-in-law is a sweep, we have 2 solid fuel stoves, both installed with his help, no liners.

beware hetas installers. on another house we installed a stove and a flue. a job i easily could have done myself but i needed a hetas certificate. lets just say it went to the eve of legal action to get them to correct all the massive installation errors and bodges that they made… big company too.