Preheat the oven to 230C/450F/Gas 8 and grease a 900g/2lb loaf tin with vegetable oil.
Add the plain flour, bicarbonate of soda and salt to a large bowl. Stir through the wholemeal flour and sugar.
Pour in the stout and buttermilk. Mix well to form a wet dough.
Tip the dough onto a lightly-floured work surface. Roll and fold the mixture gently until it forms a fat sausage, roughly the size of the tin.
Place the dough into the greased loaf tin and, with wet hands, pat down and smooth the top into the edges. Set aside for 10 minutes to rest.
Bake for 10 minutes, then reduce the oven temperature to 180C/350F/Gas 4. Bake for another 25 minutes, or until the top is golden and the base sounds hollow when tapped.
Cool in the tin for five minutes and then turn out onto a cooling rack. Let the loaf cool before slicing.