Sift flour, salt and baking powder into a bowl. Cut butter into pieces, Stir butter into flour lightly with a knife. Add lemon juice and water, and mix into a dry dough. Knead lightly on a floured board. Roll out to a thin strip.Fold in three, seal ends and turn. Roll out pastry thinly and cut into 8 squares or circles. Place 2 tablespoons apple on each piece of pastry. Moisten edges of pastry and fold in half. Press edges together firmly. Brush over with egg and milk, and sprinkle with castor sugar. Bake in a moderate to hot oven for l5 to 20 minutes or until golden brown.