I'm having some problems with this
Here's the recipe I tried
https://www.bbcgoodfood.com/recipes/7661/crab-apple-jelly
I make CAJ every year - we have a tree in the garden which produces many kg every year. That's the recipe I use and I always get a good set, jelly, not runny or CAJ-crete! I hate seeing good fruit go to waste, but there's no way I could find room for all the jars I could make, let alone eat it all or give it all away before next year.
I think the key is enough lemon juice, I use about 2 lemons per jam kettle, and use a thermometer. Wait, wait, wait until it gets to 105C, then give it a bit more, then jar up.
I can rely on my CAJ to set well, but getting a reliable set on my marmalade and blackberry jam can be a bit hit and miss. I've learned to watch the boil as it gets to 105C, there's a distinct change in the character of the boil, it goes clearer, a bit thicker, and is somehow distinct from the preceding boil character. Once you've spotted that change, a good set is much more likely, IME.