Thanks for the feedback and photos you sent us.  It sounds an expandion issue, the fact the fireboard was fastened to the brick wall by cement means the fire board with very minimal expansion properties is firmly fastened against brickwork with higher expansion movement. We do sell many of these boards and if used in the correct manner they do not crack. Fire board does also go very brittle when it has been 'fired'.  We would recommend there to be movement between the two surfaces to allow for expansion. We have spoken and discussed other options for you, inparticularly to line the inside of the fireplace with small 9" x 4" x 1" fire bricks.  We also sell cast iron panels which fit together to provide a fire proof wall.  We will work with you to provide the best option at a specially reduced price.