Sift the flour, baking powder, salt and bicarb into a mixing bowl, tipping in any bran left in the sieve. Stir in the oatmeal, bran, sugar, black pepper, cinnamon, nutmeg and allspice.
Add the butter and rub it in with your fingertips until the mixture resembles breadcrumbs. Add tablespoons of the milk and stir it in well so the mixture comes together to form a soft dough. If the mixture is a little dry, add the remaining milk.
Turn the dough out onto a sheet of greaseproof paper and shape it into a log about 25cm long. Wrap the paper round the dough and roll it gently back and forth to make a smooth shape. Twist the ends of the paper together to seal. Chill the dough for about 30 minutes.
Preheat the oven to 190 C / Gas 5. Unwrap the dough and, using a very sharp knife, cut it across into slices 8mm thick. Lline a baking tray with parchment and place the biscuits on it.
Bake for about 12 minutes or until lightly browned. Transfer the biscuits to a wire rack and leave to cool completely. They can be kept in an airtight tin for up to 5 days.
Make ahead and bake when needed; this dough keeps for up to 4 days in the fridge or up to 2 months in the freezer.