You can quote me happy today
Finally, I got the use of a ramp lift and set about tackling the ride height at the rear. With an assistant, we made a small adjustment, but realised it was the wrong way as the car began to rise once started. Another quick alteration in the right direction had it lowered maybe half inch lower than it was when a customer appeared needing the use of the lift to do the tracking. I took the opportunity to drive it around the block and the difference was immediately noticeable!
With the customer served, I got back on the lift and a different assistant, who thought "slightly lever it when I loosen the clamp" meant wack it as far as you can

which, despite expecting the rear to drop to the floor actually made the car go to full height instead. But then it got worse. I couldn't get it to move at all after that, no matter where I set the clamp. Using the lever in the cabin made no difference either - the front obeyed willingly but the rear stayed high. I was panicking now as I knew it couldn't be driven home like that and they were wanting to shut up shop soon.
Thankfully, and I don't know how exactly, I got the rear to sink again and this time I took full control and using just slight adjustment, eventually got it to a "reasonable" height and locked it off. The drive home was as if someone had completely changed the rear end suspension, there was no wallowing, no jarring over big bumps, just a a well-planted rear end.
I hope to go back next week and perfect both the rear and the front settings (it now has the pitch of a speedboat instead of a dragster

).
EDIT: Funny how adjusting the height has made the headlights brighter. They light up all the road signs so much more effectively.
Incidentally, with good visibility of the underside of the car, I noticed some weepage at either end of the trailing arm, roughly where the spheres are mounted, near some coiled pipe. Anyknow know what could be leaking (it might be just veg oil that I spilt when refitting the tank sender but I doubt it)?