Line a 23cm (9 in) pie dish with pastry. Ensure that some pastry hangs over the edge of the dish.
Combine sugar and flour. Sprinkle 1/4 of the mixture over pastry in the pie dish. Heap rhubarb over this mixture. Sprinkle with remaining sugar and flour. Dot with small knobs of butter. Cover the top with pastry, and pinch together with the bottom sheet of pastry around the edges of the dish. Cut four slits in the top pastry sheet for steam to escape.
Place pie on lowest shelf in oven. Bake for 15 minutes. Reduce oven temperature to 180 C / Gas mark 4, and continue baking for 40 to 45 minutes. Serve warm or cold with ice cream or custard.