So, I have just noticed another oversight in my existing oven. There is no floor insulation! If I put my hand underneath the oven - it is boiling hot. I had only ever thought previously about the insulation on the sides and top of the oven which is actually very good. So, I will add some insulating bricks below the oven floor and see how much of an effect this has.
I think, fixing this problem, while also ensuring a good mas of refractory brick on the floor and an oven big enough so that the floor gets a rest occasionally - will give me what i need.
A final question... is there a post regarding the A-Z process of making a clay oven - which stands above all other posts? - much like Craig's Neapolitan Garage has become comprehensive?
Thank you again,