Put the flour, baking powder, salt and mustard powder into a large mixing bowl and whisk together until smooth and well combined.
Grate in the butter, then rub it in with your fingertips until it looks like wet sand.
Finely grate in the cheese, add the chives, and then stir to combine.
Mix in the buttermilk and water until the dough just comes away from the edge of the bowl; don’t handle it any more than is necessary.
Tip on to a very lightly floured surface and flatten into a rectangle about 2.5cm high. Cut out with a fluted cutter (about 6cm / 2.5" wide for 12 scones), reshaping as necessary while handling the dough as little as possible.
Put on a baking tray and brush the egg and milk mixture.
Bake for about 12 minutes until golden. Allow to cool slightly on a rack before splitting open.
For those of you who like to use American Cup measurement