Thanks for all the ideas, I used a combination of them and did just finally get it removed.
I ended up sacrificing one jaw of a puller which I ground down until I had a tiny long-armed pry-bar with a small foot on the end, small enough to slip behind the ring and rest on the nut. Then I used that inside the bore to very slightly pry up in one spot and went round and round.
Eventually there was enough of a gap that I could fit a sharpened screwdriver behind the lip (before it was almost flush) and much to my annoyance it's not a lip, it's an angled edge! Arg, not the best for prying against but went back to my internal pry bar round and round tapping it with a hammer and walking the ring out one fraction of a millimeter at a time.
Here's a picture of that annoying lip
Thanks for all the tips, on to the next task!