Photograph: Dan Jones
In a pan, mix together the juice of the clementines and ½ lemon with the caster sugar. Slowly bring to a simmer, stirring all the time until the sugar has dissolved. Increase the heat and bubble the mixture for 5 minutes until syrupy. Leave to cool, then chill.
To serve, pour a little of the chilled clementine syrup into champagne flutes, followed by a dash of Campari. Top up with chilled Prosecco and stir.
Kitchen secret: tie pretty ribbon bows to the stems of your Champagne flutes for a little added glamour.