Place the gelatine leaves in a bowl of cold water and leave to soak. Place the prepared lychees in a blender and whizz until as smooth as possible – you should end up with about 150ml lychee purée. Spoon the purée into a medium-sized saucepan along with the sugar and cream. Mix and heat gently until the sugar has dissolved. Remove from the heat. Squeeze out the water from the gelatine and stir into the cream mixture, stirring well until dissolved. Pour the cream through a sieve into a jug, pressing everything through with the back of a spoon. Divide the mixture between 4 serving glasses, cool completely then place in the fridge to set – about 1½ hours.