Preheat the oven to 220°C/425°F/Gas 7. Whisk together the pumpkin, cream cheese and treacle until mixed well and smooth. Whisk in the sugars and spices. Whisk in the eggs and single cream (half-and-half), whisking until everything is well blended.
Pour the pumpkin mixture into an unbaked pie shell, spread evenly and bake for 10 minutes. Reduce temperature to 180°C/350°F/Gas 4 and bake until a knife inserted into the centre comes out clean, about 40 to 50 minutes. Let the pie cool on a rack until room temperature. Garnish with whipped cream, if desired.