Divide the Wattie's Boysenberries and syrup between 4 glasses. Place 2 sprigs of mint into each glass. Using a cocktail muddler or the end of a wooden spoon, muddle together in the bottom of each glass. Stir 1-2 tsp lemon juice into each glass, according to taste. Top each glass with ice, add the lemon wedges then fill with soda water. Garnish with extra mint sprigs.