1. Grease 2 x baking sheets and set the oven to 220 degrees.
2. Sift the flour, salt and baking powder into a large bowl. Add the diced butter and rub in the fat until the mixture resembles fine breadcrumbs.
3. Stir in half the cheese, the mustard powder and enough milk to give a nice soft dough (not sticky). You'll need almost all of the milk.
4. Roll out to a thickness of around 2cm, then use a 5cm pastry cutter to cut into rounds. Do not twist the cutter, or the scones will be wonky when they rise.
5. Put onto the baking sheets, setting each one with a gap between the next, as they will expand slightly. Brush the tops with the remaining milk and sprinkle with the leftover cheese.
6. Bake in the oven for about 10 minutes or until golden on top. Eat hot or cold, lathered in butter.