Preheat the oven to 190 degrees C. Combine the fruits in a 2 litre ovenproof dish. Sprinkle with the brown sugar and half the cinnamon. Spoon 2 tablespoons of water over the top, then spread the fruit until it is level. (For individual crumbles, divide the fruit among six large ramekins on a baking tray.)
To make the crumble topping, rub the butter into the flour using your fingertips (or use a food processor) until the mixture is crumbly. Stir in the demerara sugar and remaining cinnamon.
Sprinkle the topping evenly over the fruit – don't press it down. Bake in the oven for 50 minutes until the crumble is nice and golden and the fruit juices have started to bubble up around the edges. (Bake individual crumbles for about 30 minutes.) Garnish with mint. Serve warm.