Basically Ginge, when they were built they were just big openings where the fire was to sit. These were about 2.5ft high, then from that height the chimeny began and the brickwork 'corbled' over(bricks start to overhang) to form the chimeny flue. Then when built, before a fire went in the back of the fire place was made smaller to suit XX fire size(usually cast iron), then once the brick pannel was built that made it smaller the rear was filled with old bricks and mortar just pushed in by hand so that there wasnt a void for heat to escape....