In a large saucepan over medium heat, add diced peaches, lemon zest, sugar and 2 cups of water. Stir to dissolve sugar and let mixture simmer – do not boil, reduce heat if needed. When peaches are very soft, mash them with a potato masher or fork to break them up. Let mixture simmer until it thickens slightly. Remove from heat and let cool. Strain the peach syrup mixture into a bowl and discard the solids. Allow to cool. Add the lemon juice to the peach mixture. Pour into a jug along with 4 cups of water or sparkling water. Refrigerate until cold. Fill glasses with ice and pour over peach lemonade. Garnish with fresh peach slices, fresh lemon slices and mint. Serve immediately.