Preheat the oven to 220C/425F/Gas 8. Put a large casserole dish and its lid into the oven.
In a large bowl, mix the flours, sea salt, sugar and bicarbonate of soda together with your fingers. Pour in the buttermilk, bringing the mixture together as a soft dough. Working quickly (the bicarbonate of soda will start working immediately), shape the dough into a shallow round loaf about 4cm/1½in thick.
Remove the hot casserole dish from the oven, dust the inside lightly with flour then lower in the dough. Cover with the lid and return to the oven.
The bread should be ready after 25 minutes. Remove from the oven and leave in place for 5 minutes before turning out and leaving to cool slightly before eating. (Soda bread is best eaten warm.)