Sift the flour, baking powder, salt and bicarb into a mixing bowl, tipping in any bran left in the sieve. Add the oats, bran and sugar, and mix well.
Rub in the butter till the mixture resembles breadcrumbs. Add 3 tbsp of the milk and stir 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 (10 in) 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 cut into 8mm-thick slices. Line a baking tray with parchment and place the biscuits on the tray.
Bake for about 12 minutes or until lightly browned. Transfer the biscuits to a wire rack and leave to cool completely.
Melt the chocolate over a double boiler or in the microwave oven. Allow chocolate to cool but not harden before dipping the cooled biscuits into the chocolate. You can choose to dip the entire biscuit, just half, or just the top. Place biscuits a sheet of greaseproof paper till set.
You can chill the dough in the fridge for 30 minutes or up to 4 hours.