Start heating your oven to 200 C / 400 F / Gas Mark 6
Sour the milk with the lemon juice. Put all dry ingredients in a bowl and rub in the butter. Add enough of the soured milk to make a dry but still moist dough.
On a floured board, shape the dough into two rounds. Take a knife and make a cross on the top of the loaves. Place on an oiled baking sheet; bake for half an hour. Serve warm.