You may find that those oats are out of date; given the low cost of decent porridge oats I'd say that its worth just getting some new ones.

I also find that I prefer non-chopped porridge oats for flapjacks; the recipe I use doesn't have any butter and replaces the sugar with maple syrup. Another difference is that I cut the flapjack right after it comes out of the oven and not after its cooled - its much easier to cut then.