Combine the sugar and water in a large pot and heat, stirring, until the sugar has completely dissolved. Add the lemongrass and simmer for 5 minutes or until slightly syrupy.
Place the cucumbers in a large bowl. pour the syrup over and set aside until completely cooled. Once cool, add the lime juice.
Strain the mixture through a sieve lined with muslin and, using a funnel, pour into sterilised glass bottles.
Serve with sparkling water, mint, sliced lemon and ice.