To make Mary Berry blueberry muffins, preheat your oven to 200°C/gas mark 6 and line a muffin tin with paper cases. In a large bowl, mix together self-raising flour, baking powder, salt, and caster sugar. In another bowl, whisk together buttermilk, eggs, vanilla extract, and vegetable oil. Add the wet ingredients to the dry ingredients and stir until just combined. Fold in fresh blueberries. Divide the mixture among the muffin cases and sprinkle with demerara sugar. Bake for 20-25 minutes or until a skewer inserted into the center comes out clean. Allow to cool in the tin for 5 minutes before transferring onto a wire rack.