I’m definitely loading the spring the right way but before I get all the way round the hooked end of the spring forces itself out and jams in between the cages.
This doesn't compute - loading the spring is winding it up, not unwinding it. If you were winding it up and the end let go, it would have gone inside. It could only be poking out if you were trying to unwind it, right?
I recall having the exact same problem once with a SLX derailleur. No I wasn't unwinding it, first thing one checks after the spring pops out. Sadly can't remember what I was doing wrong or how I solved it.
This doesn't compute - loading the spring is winding it up, not unwinding it. If you were winding it up and the end let go, it would have gone inside. It could only be poking out if you were trying to unwind it, right?