Preheat the oven to 190°C.
Place the strawberries and raspberries for the filling in a saucepan along with the sugar. Bring to a simmer over a medium-low heat and cook, stirring occasionally, until the sugar as dissolved. Spoon into the base of a heatproof, glass baking dish.
Place the butter and flour in a food processor and pulse until it resembles rough breadcrumbs. Transfer to a mixing bowl and stir through the sugar. Spoon the crumble topping on top of the fruit, then sprinkle the redcurrants and blueberries on top. Bake for 30-35 minutes until the topping is golden brown in colour.
Remove from the oven and leave to stand for 5 minutes before serving.