Meanwhile, for the syrup, place the sugar, tonic water and lemon juice in a saucepan over low heat, stirring to dissolve the sugar. Add gin and juniper berries and simmer for 5-10 minutes until slightly thickened. Blanch lemon zest in boiling water for 2 minutes, then drain and add to the syrup. Simmer for a further 5 minutes. Cool completely.