Dissolve the cornflour in the milk. Add the condensed milk, coconut milk and grated coconut. Cook over low heat until thickened, stirring constantly with a wooden spoon.
Pour the mixture into a 24cm ring tin. Let cool, then refrigerate 3 hours until set.
Make the plum sauce: in a saucepan over low heat, dissolve the sugar in the water while stirring. Add the soaked and drained prunes and cook without stirring for 10 minutes or until thickened. Remove from heat and let cool.
Unmold the pudding on a serving plate, garnish with prunes and drizzle with the syrup.