Heat the oven to 220C (200C fan)/425F/gas 7, and line a 7cm x 23cm x 12cm baking tin with paper (this is easier to do if you scrunch up the paper into a ball first).. Put the flours, salt, bicarb and oats in a large bowl
Whisk together until well combined, making sure you break up any clumps in the bicarb.
Put the treacle in a jug and whisk in a little buttermilk to dissolve – treacle has a tendency to stick to the bottom of the jug, so mix it well – then stir in the rest of the buttermilk and the stout. Pour this mix into the bowl of dry ingredients and stir just to combine, and making sure there are no pockets of flour left unmixed.
Quickly tip the dough into the lined tin and smooth the top. Scatter over a few more oats, then bake for 20 minutes. Turn down the oven to 180C (160C fan)/350F/gas 4 and bake for 40 minutes more.
Remove the loaf from the tin and put it back in the oven directly on a rack and bake for another 15 minutes, or until it sounds hollow when tapped.
Remove, transfer to a rack to cool a little, then slice and serve warm with plenty of butter.
