Put the gelatine leaves in a bowl of cold water for 5 minutes or until soft. Squeeze the leaves and discard the water. Put the Limoncello into a bowl that fits over a pan of boiling water, add the gelatine and sugar and heat until gently dissolved. Remove from the heat and allow to cool slightly, then add the Prosecco and pour into a jug. Thread the berries onto a wooden skewer, on for each champagne glass, and place into the empty glass. Very carefully and slowly, pour the jelly into 4 serving glasses until each glass is almost full (add a piece of lemon peel if you like for presentation). Refrigerate until set.