We wanted the hearth to be flush with the surrounding wooden floor (so as not to stub ones toe on as we did with the old hearth). The air grille in the centre of the hearth is a cast iron air brick, sourced from  www.castironairbricks.co.uk. If you are planning on doing tiling yourself, and like me are an enthusiastic DIY-er, don’t do what we did and choose hand-made terracotta tiles. Not only are they not uniformly square-edged, they are not flat either. Not a great plan for a hearth!