Preheat the oven to 190C, 375F, Gas mark 5 and grease an ovenproof dish. Wash and trim the rhubarb and cut into large chunks. Place in a saucepan with 2 tablespoons of water. Sprinkle over the sugar  and cook over a low heat for 3-5 minutes only, turning constantly.  Transfer to the prepared dish and set aside. In a bowl, rub the butter into the flour with your fingertips until it resembles breadcrumbs then stir in the sugar and mix well. Sprinkle half the crumble mixture over the rhubarb pressing firmly. Add the remaining crumble mixture, spreading evenly and bake in the oven for 25 minutes until crisp and golden brown.   Serve hot or cold.