Seville oranges, being closer to the ancestral form, are not only a lot more bitter and sour than sweet oranges, but are FULL of seeds. These have been boiled for almost two hours to soften the peel, and then cut in half ready to be prepped for marmalade
and in the live and learn category: it sounds odd to say, but the new thing I learned from reading Saving The Season by Steven West, is that up 'til now I've been doing my set test wrong.
To test for a gel set: put 1 teaspoon of hot marmalade on a chilled saucer and place in the freezer for 1 minute. If the puddle of marmalade has formed a "skin" that wrinkles when you push your finger through it, then you have a gel set. If not, if the puddle is still syrup, then bring the preserving pan back to a boil and cook for another minute or two. Don't overcook the marmalade.