No 3am insight. But I have had encouragement from the Unofficial Royal Enfield Community Forum - they say, basically, it takes a lot of patience. Uhm.
Today, further playing with the change mech. Then, went out to test the results of my playing and was pleasantly surprised to find I could change gear! Not good, but with a couple of tries it was possible to change UP so I extended my test ride to the nearby village of Bowden. Which gave me a long enough run to select top gear. All four gears work!
Changes down the box more fraught but with care and a couple of tries was able to make changes. An observation on the modified gearbox - 1st, 2nd and 3rd are definitely higher than before. Can quite comfortably ride around at village speeds in second. Third does not feel like over-revving when you get to 50 mph. Looking forward to when I can make smooth, easy changes third to fourth
without the huge drop in revs.
Back from that ride:
The rest of the day has mainly been short test ride, back yard, take the end cover off the gearbox, mess about with change mech, reassemble, test ride. The first ride was the best - after further adjustment, the change was worse, so went back towards what it was like before and got to well, slightly better.
Trouble is, there is nothing like a nice threaded adjuster so you can make small, controlled changes.

No obvious may to mark the present position and move things a measurable small amount. So what you do is loosen the screws that hold the change mech, attempt to twist or swivel the mechanism one way or the other, just a little bit, oops is that too much? Put it back together and go for a test ride.
That's the method. It's not right yet, I'm hoping to make the gearchange better but that might take a few more attempts. So the gentlemen on the Unofficial Community are quite right - it takes a lot of patience.
But at least I've been out on the Slug. Four times now since rebuilding the gearbox. And on the way back from Bowden, found myself laughing out loud - had almost forgotten the Slug does that to you.
Onwards and upwards.