Preheat the oven to 190 C, gas 5. Put the flour and butter in a food processor or bowl and work together until the mixture resembles coarse breadcrumbs. Stir in 100g of the sugar.
Cut the rhubarb into 2.5 cm pieces and tip into a mixing bowl. Add the rest of the sugar and the 3 tbsps flour. Mix together well and set aside for 15-20 minutes , stirring now and then, until lightly moistened.
Spread the fruit over the base of a shallow ovenproof dish and sprinkle over the topping. Bake for 45-50 minutes until the rhubarb is tender and the top is golden brown. Serve with clotted cream.