Preheat the oven to 180C/gas 4/fan 160C. Butter and line the bottom of a 1.7 litre loaf tin (see tip below). In a large bowl, rub the flour, butter and muscovado sugar together with your fingers to make fine crumbs. Measure out 5 level tbsp of this mixture into a small bowl for the topping, and mix in to it the cinnamon and demerara sugar. Set aside.
Coarsely grate the apple down to the core and mix in with the eggs and the zest. Stir the baking powder into the rubbed-in mixture in the large bowl, then quickly and lightly stir in the egg mixture until it drops lightly from the spoon. Don’t overmix.
Gently fold in three quarters of the berries with a metal spoon, trying not to break them up. Spoon into the tin and level. Scatter the rest of the berries on top. Sprinkle over the topping and bake for 1¼ -1 hour 20 minutes. Check after 50 minutes and cover loosely with foil if it is browning too much. When done the cake will feel firm, but test with a skewer.
Leave in the tin for 30 minutes before turning out, then cool on a wire rack. Peel off the paper before cutting. Will keep wrapped in foil or in a tin for up to 2 days.