Preheat the oven at 200ºC (400ºF).
Sift the flour, baking powder and a pinch of salt into a large mixing bowl. Add the chilled, diced butter and rub into the dry ingredients using your hands.
When the mixture resembles sand and there are only very small pieces of butter remaining, add the caster/granulated sugar and mix to combine.
Make a well in the middle of the mixture, add the buttermilk and blueberries and stir to combine using a rubber spatula. Once the dough starts to come together, use your hands to form a rough ball.
Tip the dough out onto a lightly floured work surface. Very lightly knead for about 30 seconds to bring the dough into an almost smooth ball but do not overwork it.
Cut in half and flatten each half into a round disc each about 16 cm (6 ¼ inches) in diameter. Using a long, sharp knife cut each disc into 8 triangular wedge shapes.
Arrange the wedges on a prepared baking sheet lined with baking parchment and brush the tops with a little milk, scatter with demerara/turbinado sugar and bake on the middle shelf of the preheated oven for 14-20 minutes, or until well-risen and golden.
Cool on a wire rack and serve slightly warm with slices of butter ready melting on top if you wish.