Place a handful of mint leaves in a mortar dish with 3tsp of Caster Sugar and pound to a paste. Transfer to a bowl and add one litre of water, stir and dissolve sugar and set aside for an hour. Strain through a fine sieve into a freezer-proof tray and place in the freezer until set.
Fill glass with minted ice cubes and extra sprigs of mint. Top with cider and serve immediately.