I can't quite follow your description. Are you saying that the existing flue is not in the chimney?
I think you will have to get a Hetas Installer to come take a look and quote to install the wood burner. It sounds like there was a gas fire at some point and the flue liner might not be suitable for a wood burner.
I have used this company in the past..
http://specflue.com/..but might be best to use a local company.
Most reputable stove manufacturers seem to recommend lining old chimneys as this helps the wood burner draw better and prevents Carbon Monoxide leaking into bedrooms through corroded bricks and mortar in the chimney.
Regarding the hearth..
If you choose a wood burner that has feet/legs and is certified as "incapable of raising the temperature of the hearth over 100C" then you might not need to do anything to the hearth. That's provided it's big enough. The hearth must be larger than the footprint of the stove see diagram 26 on page 38..
http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_J_2013.pdfIf it's not big enough then you may need to add a piece of granite or glass of similar. The hearth has to be at least 12mm thick so as to form a step 12mm high to discourage rugs and mats from migrating too close. You might want to make it thicker than 12mm, or at least well supported, to avoid it being cracked if someone steps on the corner?
If you want to install a fire basket or stove that might make the hearth hot (eg It's not certified as mentioned above) then you need a "constructional hearth". This is much more complicated. It typically has to be quite thick (>125mm) See diagram 25 on page 37. However you might have one that thick already?