Sieve together the flour, salt, mustard, cayenne pepper, bicarbonate of soda and cream of tartar. Rub in the butter until mixture resembles fine breadcrumbs. Add cheese. Bind together with milk to form a soft dough. Turn on to a lightly floured working surface and roll out ½ inch thick. Cut into rounds, using 2 inch plain cutter, flouring the cutter each time. Put the scones on to a lightly floured baking tray and bake in a hot oven, 425°F., Gas Mark 7, for about 10 minutes or until well risen and golden brown. Serve with butter.