Heat oven to 200C/180C fan/gas 6. Whizz the porridge oats and butter together in a food processor, or rub the butter into the oats with your fingertips in a big bowl. Stir in the flours, caster sugar, bicarb, mixed spice, 1 tsp salt, the raisins, sultanas, dates and mixed peel.
Pour over the buttermilk and quickly stir in with a round-bladed knife. Tip out onto a flour-dusted surface and gently bring together into a ball with your hands. Transfer to a flour-dusted baking sheet and scatter over the demerara sugar, pressing it into the top. Use a sharp, flour-dusted knife to cut a big cross in the top and bake for 30-35 mins until crusty on the outside. Eat warm or cold, thickly sliced, with butter.