Preheat the oven to 180C/350F/Gas 4.
For the crumble filling, heat the apples, water and sugar in a large pan over a medium heat, stirring regularly. Continue to cook gently for 12-15 minutes, or until the apples have broken down. (Add more sugar if necessary.)
For the crumble topping, in a large bowl, mix together the butter, sugar and flour using your fingertips, until the mixture resembles breadcrumbs.
Spoon the crumble filling mixture in an even layer into the bottom of an ovenproof dish. Sprinkle over the crumble topping mixture. Transfer the apple crumble to the oven and bake for 18-20 minutes, or until the topping is golden-brown and the filling is bubbling.
To serve, spoon a portion of apple crumble into the centre of each of four serving plates. Drizzle over the custard or double cream, if using.
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