To make the starter, mix the yeast and water together in a bowl. Add the 250g/9oz flour and stir until it forms a thick paste. Cover with a tea towel and leave overnight at room temperature.
To make the dough, mix the yeast with 2 tablespoons of tepid water.
Pour the 1kg/2¼lb flour into a mound on a flat surface, then mix the salt in.
Make a well in the centre of the mound. Pour the yeast mixture and the starter into the well. Gradually add the remaining water and slowly combine into a dough using your hands.
Knead the dough for about 10 minutes, adding more flour or water if necessary, until it is soft but firm and elastic. The dough is ready when it springs back to shape when you push it with your finger.
Divide the dough into two and shape each half into a round loaf. Place on a baking tray, cut a cross in the top of each loaf and dust lightly with flour. Leave to rise in a warm place for about 1½ hours until double in size.
Preheat the oven to 200C/400F/Gas 6.
Bake the loaves in the oven for one hour or until the loaves sound hollow when tapped underneath.