Hi, my chimney is in a really poor state of repair and is in desperate need of a partial rebuild. Having taken some off the cement render the stone is far to badly damaged to just render in lime. As such im gong to take it apart and rebuild it properly however the large stone that sits between the stones and the pots and creates the drip edge also needs replacing. Should this Stone be a single piece - hollowed out for the flues or is it seperate pieces? Thanks for any help