Put the zest and juice from the lemon and the limes into a bowl with the sugar and stir until the sugar dissolves. Add the cream, stir well and put into the fridge until cold. When fully chilled pour into an ice cream machine and churn until thick and frozen, transfer to a plastic box with a lid and put into the freezer until a little firmer. Remove from the freezer 15 minutes before you want to eat it. If you don't have an ice cream machine pour the fridge cold mixture into the plastic box and freeze removing every couple of hours to give it a really good mix.