For the cranberry-pecan marmalade, place the cranberries, ½ cup sugar and the water in a small saucepan. Bring to the boil. Reduce heat; simmer until the cranberries pop and are softened, about 10 minutes. Remove from heat. Stir in pecans. Leave mixture to cool.
Preheat oven to 220°C. Roll out pastry to a 30 cm circle. Use to line a 25 cm pie plate. Crimp edges.
Thoroughly combine the mashed sweet potato, evaporated milk, ⅔ cup sugar, egg, egg whites, vanilla essence, cinnamon, nutmeg, allspice and salt in a large bowl.
Spread cooled marmalade over bottom of pie shell. Pour the potato mixture on top.
Bake for 10 minutes. Lower temperature to 180°C. Bake until the centre is set, about 40 minutes. Transfer pie to a wire rack. Cool for 30 minutes. Refrigerate to cool completely.