Place the gin, campari, plums and quince jelly in the shaker, puree until smooth. Top up the shaker with ice cubes, shake well for approx. 30 secs. Fill a glass tumbler ¾ full with ice cubes, pour the cocktail into the glass through the strainer and a fine sieve.
Pour in the tonic water, decorate the cocktail with the slices of plum.