To make your starter, place all the ingredients in a bowl and add 250ml cold water. Mix together thoroughly with a spoon until you have a spongy mixture, then cover with cling film and leave at room temperature at least overnight, but up to 24 hrs if you have time.
To make the bread dough, tip the ingredients into a clean bowl and add 1 tbsp fine salt, 200ml cold water and your starter. Bring all the ingredients together to a dough, adding a splash more water if too stiff, then tip out onto a lightly floured surface and knead for at least 10 mins until smooth, elastic and springy (this will take 5-7 mins in a mixer with a dough hook). Place the dough in a clean, lightly oiled bowl, cover with cling film and leave until doubled in size – about 1 hr at room temperature, 3 hrs in the fridge (see tips, below).
Tip the dough onto a floured surface and gently shape into a round – you don’t want to knock too much air out of the dough. Dust a piece of baking parchment heavily with flour and sit the dough on top. Cover with a tea towel and leave to prove for 1 hr until doubled in size.
Heat oven to 220C/200C fan/gas 7. Place a sturdy flat baking tray on the middle shelf of the oven and a smaller tray with sides underneath. Dust the dough with flour and slash with a utility knife. Slide the bread onto the hot tray on top and throw a few ice cubes (or pour some cold water) onto the tray below – this creates a burst of steam, which helps the bread form a nice crust. Bake for 25-30 mins until the loaf sounds hollow when tapped on the bottom. Leave the bread to cool completely.