Place the water, sugar, lemon juice and zest, if using, in a small pan. Warm over a medium heat until it at most reaches a simmer (don't boil), stirring occasionally, until the sugar dissolves.

Remove the pan from the heat and set aside to cool. You can speed this up after a few minutes by placing in the fridge.

Once cool, strain the mixture, if you prefer not to have the zest in there, and pour into in a shallow container such as a loaf tin. Place the container in the freezer and take it out at roughly 30 minute intervals. When you take it out, use a fork to break up the ice crystals, pulling them in from the edge of the container. Once broken up evenly, re-freeze and repeat after another 30 minutes.

Once the mixture is close to completely frozen, fluff up one last time then scoop into glasses to serve.