Preheat oven and butter dish or ramekins. Preheat the oven to 200C/180C Fan. Butter a 9x9 inch (23x23 cm) ceramic dish or 6 individual ramekins.
For the filling. In a large deep lidded saucepan combine the apples, sugar, and the water. Cook over medium heat, stirring gently, for 4-5 minutes and then cover the pan by bringing the mixture to a simmer. Simmer, without stirring, for 5-10 minutes until the apples have softened but retained their shape. Add the blackberries to the mixture and continue to simmer until all the fruit is tender (for another 5-10 minutes). Set aside to cool.
For the crumble topping. In a large bowl, rub together the flour and butter using your fingertips until the mixture resembles breadcrumbs. Add the remaining ingredients and rub them until small clumps form. Alternatively, use a food process and pulse all the crumble topping ingredients until you obtain the desired texture. Set aside the crumble topping.
For assembling the crumble. Strain the cooked fruits using a colander remembering to collect the juices in a bowl. Spoon the cooked fruits into the buttered ceramic dish or individual ramekins. Reserve any excess fruit together with the fruit juices. Sprinkle the crumble topping evenly over the fruit filling.
Bake. Bake the tart at 200C/180C Fan for 20-25 minutes, or until the crumble turns crisp and golden brown.
Prepare syrup. Boil the reserved fruit juice in a saucepan for 3-4 minutes, or until it thickens to syrup.
Serve. Serve the tart warm in slices, with the extra fruit juice spooned over it, and accompany it with custard or cream on the side.
