Preheat the oven to 180°C/ 350°F/Gas mark 4.
Melt half the butter in a saucepan over a low heat, then stir in the flour until thoroughly mixed. Slowly stir in the cream and 225ml/8fl oz cold water. Season with salt and pepper. Stir over a low heat for 5 minutes. Remove from the heat.
Put the turkey in a lightly greased baking dish. Pour the sauce over the turkey, then sprinkle with the cheese. Spread the mashed potatoes over the cheese.
Melt the remaining butter and add to the stuffing mix, then sprinkle the stuffing over the potato. Bake in the oven, uncovered, for 45 minutes. Serve hot.