1) Preheat oven to 200C/Gas 6. Place a piece of foil on the bottom rack of the oven to catch any drippings.
2) Place the flour, baking powder, salt and bicarbonate of soda into the bowl of a food processor and process for 3 to 4 pulses. Pour the mixture into a large mixing bowl and, using your hands, work the butter into the flour mixture until about half of the fat disappears and the rest is left in pea-size pieces. Make a well in the centre of the mixture and add the buttermilk and stir with a rubber spatula or wooden spoon just until it comes together. Turn the mixture out onto a piece of parchment or waxed paper that has been lightly dusted with flour, shape into a ball and wrap and store in the refrigerator while you prepare the filling.
3) Combine the blackberries, sugar, water and ginger in a large mixing bowl. Pour the mixture into a 25cm cast iron skillet and place over medium heat. Bring this to a simmer decrease the heat to medium low and continue to cook, stirring occasionally, for 15 mins, or until the liquid is thick enough to coat the back of a spoon. Retrieve the dough from the refrigerator and gently drop it on the fruit mixture using a 30ml disher or large spoon, evenly distributing it over the top. Bake in the oven for 15 to 20 mins, or until the top is just starting to brown. Remove from the oven and allow the grunt to cool for 15 to 30 mins before serving.