Put the caster sugar and liquid glucose into a saucepan with 200ml/7fl oz water and the citrus zest. Heat slowly, stirring until the sugar has dissolved, then bring to the boil and simmer for 2 minutes, before removing from the heat. Leave the sugar syrup to infuse with the zest until it has cooled to room temperature.
Add the citrus juice, then strain everything through a sieve. Stir in the Campari (if using) then chill in the fridge for at least a couple of hours – alternatively put in the freezer for half an hour.
To turn into sorbet, either churn in an ice cream maker until frozen, then decant to a freezer container. Alternatively, put in a shallow container and transfer to the freezer.
Remove from the freezer every half hour and whisk thoroughly, preferably with a hand-held mixture, to get as much air into it as possible, until it is frozen so hard that it is not possible to do this anymore – you should be able to do this at least 3–4 times.
To serve, remove from the freezer and put in the fridge half an hour before serving.
You can make this with lemons instead – use the zest of three lemons and 200-250ml/7-9fl oz lemon juice, depending on how sour you want it. Add two tablespoons of limoncello if you want to include alcohol.