1. Place the juice in a saucepan with the water and the sugar. Heat gently, stirring occasionally, until sugar dissolves.
2. Stir in the cornflour paste and gradually bring up to the boil, stirring, until thickened. Add the clementine slices to the pan and heat for 2 minutes. Remove from heat and leave to cool.
3. Meanwhile, place the slices of ginger cake in the base of a large trifle bowl. Spoon over the ginger wine or sherry.
4. Arrange clementine slices upright all around the inside of the bowl. Spoon the remaining slices and thickened juices into the centre. Pour the custard on top.
Cover a baking sheet or tray with foil. Sprinkle 6 tbsp caster sugar evenly over the foil. Cook under a hot grill (keep watching the sugar as it can burn very quickly) for 1-2 minutes. When patches of sugar start to caramelise and turn golden, carefully tilt the tray from side to side to ensure it cooks evenly. Leave to cool, peel away foil and break into small pieces.