Wash mint leaves thoroughly and dry well. Place them in an airtight container with a litre of filtered water and keep refrigerated for 24 hours. Cold infusion, to my mind, works better than heat with mint. It imparts the fresh flavour without the “stemmy” bitterness and doesn’t leech the green from the leaves, keeping the granita white. Bring remaining 250ml water and sugar to the boil. Remove as soon as it’s boiling and let cool. Sieve mint infusion through muslin to remove any fine particles and stir into cooled sugar syrup, along with lemon juice. Place in a clean, shallow, plastic container, with a lid, and put into the freezer. After an hour, it should begin to freeze. Remove and give mixture a quick whisk so it forms small crystals rather than setting hard. Repeat every 15 minutes until it has set. Serves 8-10.