For the custard, prepare the pudding according to package directions, but use one-eighth less milk than indicated.
Allow to cool slightly, stirring frequently. Then stir in the vanilla yogurt.
For the elderflower jelly, soak the gelatin sheets in a little cold water, squeeze them out and heat them with the syrup-water mixture in which you have sprinkled some elderflowers, thus dissolving them.
Pour the jelly on the cooled pudding and let it set.