With a wooden spoon mix the first 4 ingredients in a good-sized bowl. Add the water and stir till all the flour is absorbed. Cover the bowl and leave the dough to rise at room temperature for 18 hours.
Flour your work surface. Tip out the dough, dust it with flour, shape it roughly into a square an inch deep, then fold 3 times into a strip 4 by 10 inches. Let it rest for 10 minutes. Rub flour then oats into the tea-towel. Fold the dough into a cube, smooth, then place carefully onto the towel. Cover, and leave in a warm place to rise for 2 hours.
30 minutes before the rising’s finished, put the baking dish with its lid into the oven and set at 425 ˚ F. When the oven’s heated, tip the dough carefully into the hot dish, and replace the lid. Bake for 30 minutes, then remove the lid and bake for 20 more minutes until the loaf is rich brown. Put on a rack to cool.