dome made from storage radiator bricks questions

[suffolk punch]

this is my mark two oven build, number one still works ok but loses heat too quick. I have built the dome and hearth with bricks from electric storage radiator bricks cut to shape and laid in lime/sand mortar (no cement) the radiator brick dome is about 6 inches thick and i have covered the exterior with two layers of broken clay tile laid in the same mortar to cover any cracks that might occur. My mark one model was built the same but with normal clay bricks, no cracking but dosn' retain its heat long enough. Has anyone used these bricks? do i need any more thermal mass before i put on the insulation? i used ceramic blanket for mark one, is this necessary? would fibreglass loft insulation do as well? what oven tempratures would i expect say 3 hours and 8 hours after the fire dies down?