Sift the flour and bicarbonate of soda into a large mixing bowl and stir in the salt. Make a well in the centre and pour in the buttermilk, stirring as you go. If necessary, add a tablespoon or two of milk to bring the mixture together; it should form a soft dough, just this side of sticky.
Tip it out on to a lightly floured work surface and knead lightly for about a minute, just long enough to pull it together into a loose ball but no longer – you need to get it into the oven while the bicarb is still doing its stuff. You're not looking for the kind of smooth, elastic dough you’d get with a yeast-based bread.
Put the round of dough on a lightly floured baking sheet and dust generously with flour. Mark a deep cross in it with a sharp, serrated knife, cutting about two-thirds of the way through the loaf. Put it in an oven preheated to 200°C/gas mark 6 and bake for 40-45 minutes, until the loaf sounds hollow when tapped underneath.
Cool on a wire rack if you like a crunchy crust, or wrap in a clean tea towel if you prefer a soft crust. Soda bread is best eaten while still warm, spread with salty butter and/or a dollop of your favourite jam. But if you have some left over the next day, it makes great toast.