Sift the flour, baking powder and bicarbonate of soda into a mixing bowl. Add the oatmeal, bran and mix well to combine.
Cream together the margarine, Natvia and egg, beat well allowing plenty of air through the mixture.
Add the margarine mixture to the flour mixture and rub the ingredients together with your fingertips until it resembles breadcrumbs. Add 3 tablespoons milk and stir in well so the mixture forms a soft dough. If the mixture is a little dry, add the remaining 1 tablespoon milk.
Turn the dough out onto a sheet of plastic wrap and shape it into a log about 25cm long. Wrap the plastic around the dough and roll it gently back and forwards to make a smooth shape. Twist the ends of the plastic together to seal. Chill the dough for about 30 minutes. It can be kept for up to 4 days in the refrigerator.
Preheat the oven to 190 degree. Unwrap the dough and, using a very sharp knife, cut it across into slices 1cm thick. Use greaseproof paper to line a baking tray and place the dough slices 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 container for up to 5 days.