When you do build this oven try and get some more info on types of mortar.
I'm guessing you wold use a lime mortar rather than cement.
There is also a British website that deals with period properties the contributers Know quite a bit about
'ye olde wayes'of construction.(Forum)
There is a Welsh website that has removed old properties some being donated from around Wales
(some were in danger of being of being demolished and lost forever)
The Museum of Welsh Life.
For lime mortars etc.
Building and Using a Medieval-Style Hemispherical Bake Oven,
That all I know for now let me know if you need any other info