Installing a gas fire in an existing fireplace

Hi, I have an existing fireplace that houses a real fire although I have never used it. I want to install a gas fire in the opening. Given that it has a proper chimney, do I need a flue liner? reading the blurb from the fire manufacturers, I would say I have a Class 1 flue (ie existing chimney) which does not appear to need a flue liner. I asked my builder if he knows a CORGI guy as it also needs a gas supply to be run to it under the floor which the builder is about to level. He stated that I would need a flue. Any ideas what the criteria are. Due to access to the chimney (ie difficult) I would prefer not to have the hassle and expense. Could I install a fan flue to the outside as structurally this would not be a problem?
Thanks for any advice.

First thing you should do is to get a smoke test done on it, if it passes that then you will have to sweep the flue & knock out all the old fire bricks & make a proper builders opening, usually 12" X 18" but if you are using an inset fire check the dimensions for it.
One more thing if there is more than one pot on the stack, & you want to be really sure the flue is OK, the get someone on the roof & put a bag over the pot you are testing & see if any smoke comes out of any of the others, if it does then you will need a liner fitting.

Thanks keego. The existing opening is already quite large. The fireplace is quite large and opens into a 25' x 25' room. In fact one of the problems is getting a fire large enough to not look daft. I do want to go with an inset so I guess my builder will need to reduce the opening? The actual type my wife wants is the cast black old fashioned variety as we have one of these in our dining room. Does the cast backing cover a large hole? Also thinking of a limestone (or look alike) for the mantel. Anything I need to look out for there?

An opening that has been used with solid fuel will be huge by the time you take out the old fire bricks etc, you will need to reduce the opening with new brickwork & then plaster the wall.
Don't be tempted to use the old opening as it is.