Preheat the oven to 200C (180C fan). In a large bowl, mix the flours with the bicarbonate of soda, salt and sugar. Whisk to combine. In a separate bowl, mix the buttermilk with 150ml water, pour into the bowl of dry ingredients and stir to combine. Dust the work surface with extra flour and knead the dough into a large round loaf (it will be quite sticky). Place on a lined baking tray. Use a sharp knife to cut a deep cross in the top. Brush with the milk and scatter the porridge oats on top. Place in the oven for 40 minutes and remove when the top is crusty and when you flip it over and tap the base it sounds hollow. Set aside on a wire rack to cool.