To make apple puree, wash apples then chop into rough chunks, pips, skin and all. Place in a saucepan and add castor sugar. Add water and cook until apples are soft and starting to fall apart. Let mixture cool for a few minutes before passing it through a fine sieve into a bowl, then add glucose and mix well. Place bowl in the fridge to chill. Once chilled add lemon juice. If you have an ice-cream maker, place the mixture into it and churn according to instructions until frozen. If you don't have an ice-cream maker, place the mixture in a shallow tray and put in the freezer. Remove it every hour or so to give it a good stir to break up the ice crystals. Once frozen, pile spoonfuls of sorbet into a glass and top with a sprig of mint.