Preheat oven to 180 fan. Grease and line a 900 g (2 lb) loaf tin.
Sift the plain flour, salt and bicarbonate of soda into a large bowl. Stir in the wholemeal flour, oats and brown sugar. Make a well in the centre.
Mix the buttermilk, Guinness and melted butter in a jug, pour into the dry ingredients and stir to combine. The mixture is a batter.
Pour into the prepared loaf tin and sprinkle with some of the wholemeal flour.
Place on the top shelf of the oven and bake for 55-60 minutes until lightly risen and hollow sounding when tapped underneath.
Remove from the oven and allow to cool slightly in the tin, then turn out onto a wire rack and wrap in a clean muslin or tea towel to cool further before serving.
