Method

To make the ice cream, mix the zest, sugar and cream. Freeze overnight. Whisk in the lime juice and freeze again.
To make the syrup, combine all the ingredients in a small saucepan over a medium heat. Reduce it till it coats the back of a spoon.

To make the tonic jelly, soak the gelatine leaves in cold water for one minute. Place the sugar and tonic in a pan and bring to the boil. Take off the heat.
Take the gelatine out of the water and squeeze out all excess water. Add the gelatine to the tonic. Set in a container and in the fridge for 24 hours.

To serve, cut blocks of the tonic jelly, add a scoop of the lime ice cream and a drizzle of the syrup.