Put 125 ml/ 4 fl oz hand-warm water in a bowl and add half the yeast. Let stand for a minute, then stir with a wooden spoon until the yeast dissolves. Now stir in 115 g/ 4 oz strong white bread flour and the bran flour until you have a thick, smooth batter, adding a little more water if needed. Cover the bowl with clingfilm and leave at room temperature for 24–36 hours, when it will be bubbly, risen and smell strongly of yeast.
Transfer this to a large mixing bowl or to the bowl of an electric mixer. Break it up with a wooden spoon, then add a further 600 ml/ 21 fl oz hand-warm water, the remaining yeast, 200 g/ 7 oz more strong white bread flour, half the wholemeal flour and the sea salt. If using a mixer, turn on the dough hook at slow speed. Check manufacturer’s instructions to make sure your machine is up to this task! If the motor burns out, don’t say you haven’t been warned. Gradually add the remaining white and wholemeal flours. After 5 minutes, increase speed to medium and work for a further 5 minutes, when the dough should be springy to the touch and have a satin sheen.
Remove to a floured surface and continue kneading by hand for a final 5 minutes, shaking on flour if it is too sticky to work with.
Roll the dough into a ball and put it in a bowl lightly greased with olive oil. Brush the surface with a little more oil and cover with a clean damp cloth. Put to rise in a warm place for 2–3 hours, until doubled in volume. The ideal temperature is 23–26°C/74–78°F.
Knock the dough down, turn it over, cover again and leave for 30 minutes.
Turn the dough out, divide into 2 and flatten, then reform into 2 balls. Line two 20-cm / 8-inch bowls or baskets with clean drying-up cloths and flour them heavily. Put the dough into them and dust the tops with flour. Leave to rise again for 2 hours, until almost doubled in volume.
Preheat the oven to 230°C/450°F/ gas 8. Turn the bowls upside down to turn the loaves on to a heavily floured board. Slash the tops with a razor blade to a depth of 5 mm / ¼ inch. Spray the insides of the oven with water, close the door and leave for steam to generate for 3 minutes. Ideally you want to slide the loaves on to a baking stone in the centre of the oven. If this is not possible, slide the loaves on to a preheated non-stick Swiss roll tin.
Bake for 10–15 minutes then lower the temperature to 200°/400°F/gas 6 and cook for further 30 minutes. Test by rapping the bottom of the loaves as above.