Juice the watermelon in an electric juicer and then the strawberries.
Place the sugar and water in a medium saucepan and heat until sugar is dissolved and just simmering.
Add the watermelon and strawberry juice to the syrup (strain the watermelon juice, to avoid adding the foam on the surface) and bring to the simmer again. Then switch off the heat and add the mint leaves. Cover and leave to stand for about 2 hours. The mint will infuse with the syrup. Refrigerate until ready to use.
To serve, mix the syrup with the soda water, in individual glasses,as you would with cordial syrup, adjusting the sweetness to your liking. Garnish with fresh strawberries.
Place the remaining syrup in a glass jar or bottle in the refrigerator.
Note: Makes about 1 litre.