Next time I am going to use refractory (heat-proof) bricks and refractory cement for the inner dome and then make an insulation later from someting new and interesting like rock wool as used for house insulation. I also found that the terra-cotta tiles on the baking surface shaled a little so I might use refractory bricks for that too.
You absolutely have to put a waterproofing layer like roof tiles over the whole thing or it will start to disintegrate after a few rain seasons (Britain has one continuous rain season, come to think of it it...). It lasted 3 years without a waterproof cover and was occasionally patched. And then one day after a heavy rain.... well, you can the guess the story's ending! Suffice to say, it is not more.