In a large saucepan, mix together caster sugar, 4 tbsp flour and salt. Stir in cranberries and maple syrup. Cook over high heat, stirring constantly. When mixture comes to the boil, reduce heat, cover, and simmer 5 minutes, stirring occasionally.
Stir apples into simmering mixture, and continue to cook for 5 minutes, or until apples are tender. Remove from heat, and stir in walnuts. Pour apple mixture into pastry case; set aside.
In a medium bowl, combine breadcrumbs, 100g flour, brown sugar and melted butter. Mix well, and sprinkle over apple filling.
Bake 30 minutes in the preheated oven, or until topping is golden brown. Serve warm.