Finely grate lemon zest from the lemons into a pan, then add sugar, 250ml cold water and basil leaves. Warm over a low heat until the sugar dissolves, about 5 minutes.
Strain the mixture through a fine sieve into a bowl, returning half the lemon zest to the liquid (discarding the rest).
Squeeze in the lemon juice, removing any pips.
Stir well, allow to cool slightly, then whisk the mixture by hand for about a minute. Pour this mixture into a baking pan and chill in the freezer for around 1 hour
Take the pan out of the freezer. Scrape the mixture with a fork and return to the freezer.
Repeat this process every 30 minutes for a total of 3 hours.
The granita is done when the mixture is completely frozen and appears dry and flaky in texture.