Combine the silky bag with 100mlof boiling water and infuse for 5 minutes. Discard the silky bag, then add the caster sugar to the tea and stir until dissolved. Pour the syrup into a sterilised bottle and keep in the fridge.
Fill a highball glass with ice. Add 35ml of the Sour Cherry and Orange syrup (save the rest in the fridge for another time), the citrus juice and soda water. Add the Cognac, if using. Stir to combine and garnish with the maraschino cherries.