Slice the watermelon and carefully remove the seeds and the rind. Put the fruit in a processor and whizz until it makes a smooth purée
In a pan, dissolve the sugar into 200ml (7fl oz) of water, bring to the boil and simmer for 2 minutes. Remove and allow it to cool.
When cooled thoroughly, mix the liquid with the watermelon purée and put in the freezer.
Then mix every 15 minutes for 2 hours, and then every half an hour until the granita appears grainy and all the crystals have broken - it'll look like snow or coarse sand.