Photo: Dan Jones
In a pan, mix together the clementine and lemon juice with the caster sugar. Slowly bring to a simmer, stirring all the time until the sugar has dissolved. Increase the heat and simmer 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.