Stir orange juice, lemon zest, lime zest, lemon juice, lime juice, demerara syrup and Pimm's together in a large glass measuring cup. Cover with plastic and refrigerate at least 2 hours, or up to 24 hours. Strain mixture through a fine mesh sieve into two glasses filled with ice. Top with club soda and garnish with extra slices lime or lemon.

Fill a cocktail shaker halfway with ice. Add orange juice, lemon juice, lime juice, demerara syrup and Pimm's (omit lemon and lime zest) and shake well. Pour into two glasses filled with ice. Top with club soda and garnish with extra slices lime or lemon.

Pour 3 cups orange juice, ½ cup lemon juice, ½ cup lime juice, 1 cup demerara syrup and 2 cups Pimm's to a large pitcher or bottle (recipe makes almost 2 quarts/liters, so choose your container accordingly). Refrigerate at least 2 hours until chilled. Pour into glasses filled with ice leaving about ¼ of the glass empty. Top with club soda and garnish with extra slices lime or lemon.