Tip all the cake ingredients into a large bowl and stir into a smooth, thick batter. Spoon half of the batter into the cake tin, and scatter on half the sugar/cinammon topping mix, then repeat. (The second dollop of mixture might not look like there will be enough to cover the first, but blob it out fairly evenly and coax it to the corners with the back of a clean teaspoon and it will be fine!). Finally pour over the saucepan full of melted butter and bake for 30-35 minutes.