I was cursing the idea of using the flints after three days of hard work, but now I'm really pleased with it. It's a very cheap and appropriate way to finish the oven. These days it's an expensive way to build (due to the labour costs), but around here it's a method that's been used for at least the last couple of hundred years, as the building material was free.
I got a bit nervy after the first foot or so, as I was concerned about the mortar bonding tightly to the thermalite blocks. To help the bond I screwed 2" screws into the blocks every 6", leaving about 1" sticking out that I embedded into the mortar. All finished on the masonry work now - just need to leave this to cure, then clean everything up.
Looks good in the summerhouse, especially now I've finished the front and side boarding off. Just need to do the back now, then lay the floor.