Put the sugar in a saucepan with 250ml (8fl.oz) water and heat, stirring until dissolved.
Bring to the boil and simmer for 5 minutes. Remove from the heat and leave to cool.
Place 45ml (3 tbsp) water in a small heatproof bowl. Using a pair of scissors, snip the Fine Leaf Gelatine into the water. Leave to soak for about 10 minutes. Stand the bowl over a saucepan of simmering water and heat gently until dissolved. Alternatively, heat in the microwave for about 25 seconds on High. Do not allow to boil as this prevents a proper set taking place.
Mix the gin, tonic water and Gelatine together with the sugar syrup, and pour into 4 drink tumblers.
Chill for about an hour until just beginning to set, but still soft in texture. Garnish with Gin and Tonic accompaniments. Serve straight from the tumbler.