@Tscarborough - thanks for the info, that's reassuring. I've been reading up on the various types of brick. I've just picked up a ceiling's worth of fire brick, exactly the same spec as the floor. The next bake will use them.
Out of interest, why are the bricks with holes bad? The ceiling that I used for this bake were bricks with bout 8 holes in each brick - presumably to make them able to be reinforced.