Made a start on the base today. I'm building it out of some waste concrete from the garden (a demolished concrete wall made of breeze blocks, plus some concrete steps) and some rockery stones that I got from freecycle. Total cost = £0 (so long as you forget about the mortar and cement...). I'm using a 2:2:1 sharp sand:builders sand:cement for mortar, as I want a stiffer mortar that will be strong. The rockery stone will only go around the front, since only the front will be visible when it's finished. I'm planning to have some concrete blocks in the middle of the circle to support the oven hearth slab - the walls I'm bodging together here won't be very strong, and I don't want to have to rely on them for structural integrity. I'll start adding these tomorrow.
This is slow work, and it's fiddly trying to get stones to fit in gaps. I'll probably fill the spare space in the base with rubble and subsoil from elsewhere in the garden. I'm saving doubly here - to get rid of all this concrete and subsoil I'd need to hire a skip, and the cheapest you can get a decent sized one for round here is £130!