Place the apple pieces in a saucepan with a little water, the caster sugar, the cinnamon quill and the lemon zest. Cook until the apples are tender.
Meanwhile combine the flour, brown sugar and rub the diced butter through until you have a consistency that resembles large breadcrumbs.
Place the apples and their juices in an lightly greased oven proof dish, cover with crumble mix and cook for 30 to 35 minutes or until the crumble is a lovely golden brown.