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Preheat oven to 180 degrees Celsius.
Put the rhubarb into a 1.2 ltr ovenproof dish. Sprinkle over the water and caster sugar.
Sift flour into bowl, add the butter and rub in with your fingertips until the mixture resembles breadcrumbs. Stir in the demerara sugar.
Spread the crumble mixture over the rhubarb - don't pat it down too much.
Bake for 35 - 40 minutes or until the top is golden brown and the rhubarb starts bubbling through at the edges.