Peel, seed and chop 6 English cucumbers. Puree, then strain through a sieve into a bowl, pressing to extract the juice. Chill. Combine 1/3 cup lemon juice, 8 sprigs mint and 1/4 cup sugar in a pitcher; muddle with a wooden spoon. Stir in 2 cups gin, the cucumber juice and some cucumber and lemon slices. Divide among 6 to 8 ice-filled glasses.