In a large mixing bowl, combine the flours, salt, sugar, butter, yeast and 300ml (1/2pt) warm water to form a dough. Knead, by hand, for 10 minutes, until smooth. Cover the bowl with a piece of lightly oiled clingflm and leave to rise in a warm place for 1 hour, or until doubled in size. Grease a 1kg (2lb) loaf tin. Peel and finely chop 1 apple. Tip out the dough onto a lightly floured surface and knead for 2 minutes. Fold in the chopped apple and hazelnuts and knead until evenly distributed. Transfer the dough to the prepared tin, cover with lightly oiled clingfim and leave to rise for 1 hour more. Preheat the oven to gas 6, 200°C, fan 180°C. Finely slice the remaining 1/2 apple, leaving the skin on. Brush the apple slices with milk and arrange in a line on top of the dough. Brush again with milk. Bake the loaf for 30 minutes, then remove from the oven and cover with foil (this will protect the apples against browning too quickly). Return to the oven for a further 15 minutes, until the loaf is risen and golden. Top tip: If the loaf sounds hollow when you tap its base, it's cooked through.