Peel apple, remove core and grate coarsely. Sieve together flour, salt and baking powder. Rub in butter until mixture resembles fine breadcrumbs, then add caster sugar and apple. Mix to a soft, but not sticky, dough with a scant ¼ pint milk. Roll out the dough on a lightly floured surface to an 8 inch round. Place on a greased baking sheet, brush the top well with milk and sprinkle with the demerara sugar. Mark into 8 wedges. Bake in a moderately hot oven, 400°F., Gas Mark 6, for 20–25 minutes or until golden brown.