Right, so that resistor chain has been replaced, along with the 1000pF disc cap (C90).
Also, the diode and cap (C80) in the 80 volt secondary loop have been replaced.
For good measure I thought hang it, I'll also replace the last 2 caps on the EHT circuit, C78 and 79, the 0.033uF high voltage ones (are they part of the doubler circuit?), and it was then I may have discovered the original problem. Where C78 joins ground is a rather inaccessible area of the board due to the double coil arrangement. After some difficulty in replacing the cap, I found a track lifted adjacent to the joint, so that there was no connection to this end. Could this have been the original problem? Who knows! (Perhaps I lifted it in the repair). There certainly seem to have been several areas out-of-spec in the course of this saga. Really though, it wasn't until I replaced the big rectifiers that I started getting somewhere.
Anyway I'm happy to report that after all these measures, all voltages are pretty spot on and stable, except the phosphor display line which seems content to sit at +2500 volts , rather than 3000 as specified. However, the unit is behaving generally very well, calibrates nicely eliminating pin-cushion etc, and even a few timebase anomalies seem to have sorted themselves out! I celebrated by making twin Lissajous displays dancing off the mains frequency for a while..
It's going to be nice having an analogue scope once again - there is something so immediate and responsive about them, that my Rigol can't match. And the fact that the aesthetics are from a 1950's US submarine are just an added bonus!
Many thanks team (especially Will)!