Pour the cream and milk into a large pan, add the popcorn and bring to a gentle simmer, pushing the popcorn under the liquid and squashing gently on the bottom of the pan. Bubble for 1 min, then remove from the heat, transfer to a jug and chill for at least 6 hrs, or preferably overnight. Strain the popcorn cream back into a clean pan and gently reheat, discarding the remaining bits of popcorn. Meanwhile, place the gelatine leaves in cold water to soften for 3-5 mins. When the popcorn cream is steaming and the gelatine is soft, remove it from the water and squeeze out any excess drops. Place in the hot popcorn cream and stir until dissolved. Set aside to cool a little.