Your right to worry about the pipe in the wall at present, so to put in a new fire go back to where you know the gas run is sound and safe ie under the floor. Run a newgas feed ensuring it is the correct size iaw regs. If you run through any brick work etc sleeve the pipe as you already mention. If you have to break through a fire box, fire cavity etc, you must ensure you leave the openings are sealed with grommets etc. Have alook at the registered gas engineer mag re chimneys feb issue for some help. Remember to tape up gas pipe within the flue to prevent corrosion etc. Have a look on a flue box manufacturer'e site for instructions on how to install them ie selkirk.
Dont worry about the attitude of some on this site, if you have your acs and need help ask, no one knows it all and fires can be a bu**er, and having seen a few recently, installed by what would have been corgi engineers, they obviously hadnt had the courage to ask about flue boxes and fire openings or else they would have done the jobs properly.
Not being offensive, but just my opinion of some of the more "experienced" engineers out there.