Roll out the pastry thinly and use half to line a pie plate. Cover with the oatmeal and pour on the treacle. Sprinkle the raisins evenly over the tart. Moisten the edges of the tart with water and cover with the rest of the pastry. Press the edges together to seal, and trim off any surplus pastry. Flute the edges. Bake at 425°F, 220°C, gas mark 7 for twenty-five minutes. Serve hot or cold.