Squeeze the juice of the clementine quarter into a cocktail shaker, then add the quarter.
Add the cranberry jelly and the sloe gin, top up with ice cubes and shake well.
Pour the champagne into a flute glass until about two-thirds full.
Strain the clementine, cranberry and sloe gin mixture into the flute glass.
Garnish with the sprig of sugared redcurrants.
