Combine the pumpkin, single cream, milk, ginger, nutmeg, cinnamon, cloves, salt and 4 tablespoons sugar in a liquidiser. Blend until smooth.
Pour the cold water into a small bowl and sprinkle gelatine over to soften.
Combine the egg yolks and the remaining 100g of sugar in a large saucepan. Use a hand mixer to beat the yolks and sugar until they become thick and pale, about 5 minutes. Mix in the blended pumpkin puree and heat gently over low heat, stirring, just until the mixture is hot and small bubbles begin to form. Do not boil. Spoon pumpkin mixture into a large bowl and stir in the vanilla, whisky and gelatine mixture. Refrigerate the pumpkin mixture until it begins to set, about 20 minutes.
Use a hand mixer to whip the double cream in a large bowl. With a rubber spatula or wire whisk, fold 1/3 of the whipped cream into the cooled pumpkin mixture. Gently run the spatula through the centre of the bowl, then around the sides of the bowl, repeating until fully incorporated. Add the remaining cream, folding just until incorporated.
Pour mixture into prepared pastry case and chill until set, 2 or 3 hours. Garnish with additional whipped cream, if desired.