Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the cranberries, plums and grapes into the slow cooker pot.
Mix the cornflour with a little of the grape juice in a bowl until smooth, then stir in the remaining juice. Pour into the slow cooker pot and add the sugar, cinnamon and orange rind. Stir together, then cover with the lid and cook on low for 2½–3½ hours or until the fruit is tender.
Stir, discard the cinnamon and orange rind and serve warm or cold spooned into bowls and topped with the cream folded into the lemon curd.