Preheat oven to 180°C/350°F/Gas Mark 4. Mix berries, cherries and apples together with half the sugar. Place in a greased 24 cm (9 ½ in) ovenproof dish. Using a food processor or your fingers, rub butter into flour to resemble breadcrumbs and mix in remaining sugar and hazelnuts. Sprinkle the crumble mixture over fruit and dot with the extra butter. Bake until the crust is golden and the juices begin to seep through.