Pour the gin, lemon juice and sugar syrup in a cocktail shaker and fill up with ice cubes. Shake well until the outside of the shaker feels cold then strain into a tall glass filled with more ice and top up with sparkling water. Garnish with a lemon slice.
How to make sugar syrupTo make sugar syrup put 200g white caster sugar into a saucepan with 100ml water. Heat gently until the sugar has dissolved and it starts to simmer then turn off the heat and allow to cool before using. Store in a sterilised bottle for up to 1 week.
For a heady frothFor added flair put the gin, lemon juice and sugar syrup in the cocktail shaker, but instead of adding the ice, put in 1 egg white. Shake really well to froth the mixture, then add ice and shake again. Double strain before adding the sparkling water.