I was lucky with the hearth bricks, there are no edges that stick up and all the gaps between the arch bricks are tiny (planning and swapping lots of bricks round more than luck)
I used ready mixed mortar for the small gaps and a fire clay mix for the bigger gaps and back filling.
Here's how far I have got now...
I couldn't resist a small fire to get some heat into it before I cover the oven.
Bit of an attempt to get an air gap between the oven and the front wall, which is now filled with vermiculite.
At the moment the oven bricks are covered with a layer of render, the lower rear wall is rendered (I'm having to do that in 2 parts) and the front arch has been made.
I'm now starting on the chimney, insulating the oven and building the side walls up.