Beat together the butter, sugar, mixed spice, eggs, half of the flour and the baking powder in a bowl until pale and creamy. Peel 3 of the apples and coarsely grate into the bowl, stirring after each one is grated. Stir in the remaining flour.
Spoon the mixture into a greased and lined 32 x 22 cm (12½ x 8½ inch) shallow baking tin (see
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 40–45 minutes until golden, just firm to the touch and a skewer inserted into the centre comes out clean. Sprinkle with caster sugar and leave to cool in the tin. Peel off the lining paper and cut into squares, then serve, warm or cold, with Spiced Blackberry Sauce, if liked (see below).