Start by making the juniper syrup. Put the sugar in a small saucepan, then add 100ml water and the juniper berries. Bring to the boil, then take off the heat and gently squash the berries in the liquid using a potato masher. Leave to steep until completely cold, then strain into a sterilised bottle or jar. Will keep in the fridge for up to 2 weeks.
Put the sloe gin, lemon juice, gin and 2 tsp of the syrup in a cocktail shaker with a few ice cubes. Shake well and strain into a tumbler filled with crushed ice. Serve immediately.