It just kind of happened, totally unexpected.
So what's the key?
Perhaps that there isn't one.
I'd decided to work hard at coming up during my holiday a week from now. Until then I'd planned on laying some groundwork, Drop back everyday, work on breathing and the rocking back and forth. That's pretty much what I was doing here but on that third rock forward I came up a little more than I expected and it felt good, lighter, so I just went for it and up I came. Perhaps you can tell that I was rather pleased about it. (bit embarrassed about that bit, not very Zen of me, but couldn't edit it out without cutting short the coming up).
I think attempting to come up after dropping back is to be recommended. I've been dropping back everyday this week and have become much more confident with it. The more you do the slower it all seems to happen and the more time you have to feel what's going on. Also I haven't felt any strain on my back at all this week and when I came up just now it was pretty much effortless. In contrast, last month when I was trying to come up from UD without the drop back I felt a lot of strain in my back and it was as if I was trying to force myself up.
Excited about it, but looking at the video it's untidy and lacks finesse, hardly any control to it. Still lots of work to do, want to be able to hang back with my arms over my head and breathe and perhaps come up hands over head too.
Thank you to everyone for all the advice, suggestions and encouragement in your comments