It all depends on what type of chimney you have if its class one etc if you fit a cowel, do you know what type?has the chimney passed a flue flow test.?
does the fire require additional ventilation?nothing to do with question but things that need to be assessed.
frankly if you cant answer these,which by your post you cant,then please contact a corgi registered plumber who can do all the relevant tests advise you what he can offer and fill in a GWN which needs to be completed. I am not trying to be funny but as a registered gas engineer I would be failing in my duties if I advised you any other way.
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