Dumb question, but was their function in the heater to act as thermal mass to store heat, or as insulation? I can't believe that a normal storage heater would produce temperatures high enough to require fire brick insulation. Also, there are many kinds of "fire brick". Some are very light and full of air, like mineral polystyrene, and are used for high temperature forge liners. Others are a little stronger but less insulative, and still others have almost no real insulation properties, but can take the temperature, these last feel heavy enough to be bricks.