For the sake of a relatively small amount of cash, personally, i would go with a liner.
A lot of the cost of a 'kit' are the adapters etc and some of them will be needed either way. My opinion is that, with a liner, the job is going to be easier to sweep properly, and, even if its not swept, there's more chance of surviving a chimney fire.
I smoke tested our stack before lighting the existing open fire, but still fitted a liner when i put in the log burner. Again, its just personal opinion though - i know plenty of installs use the existing stack, unlined.
Not the best photos, but the only ones to hand (not missing an opportunity to post photos and all! ; )
This -
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To this -
(yeah, i know its not complete in the photo, but its all i have to hand)
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And this -
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To this -
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whats the pooch a whippet ??