By now the archway had dried and was safe to stand on…
We have added a 4inch thick vermicrete layer to insulate the oven.
We had managed to get some roof tiles free, that were left over from a project, so used those to build a roof, bedded on the insulation layer using 5:1 mortar mix and nailed in.
A few days later I added some cheap ridge tiles (£3.50 each from a Roofing supplies shop)
I then looked for a chimney pot to finish off the top – they are ridiculously expensive £50 in Travis Perkins. A property developer friend was demolishing an old house and gave me one! You can often pick them up on ebay for around 20 quid.