Bring the water and sugar to a boil in a pan until the sugar has dissolved. Wash and finely slice the cucumber, add to the pot with the basil leaves and leave to infuse for about 10 minutes over low heat.
Leave to cool, then strain.
Mix the water mixture, tonic, lemon juice and gin and pour into ice molds.
Place finely sliced cucumber in the molds.
Place in the freezer overnight and enjoy it the next day!