[Diabolical Artificer]

Right, I've done some more work on this and tried a few circuits. First off I built the anode/screen supply out of the Sussex. Essentially its the same as the one I built but it has a CCS supplying a zener string, this gives us a hopefully stable reference voltage, this goes into a voltage divider, which sets gate/OP voltage.

It worked pretty much first time, but like mine isn't stable, OP V drifts a little as the circuit warms and V out drops with increased current demand.

Next I tried a few floating regulator types, a few based on the LM317 application notes on HV voltage regulator's, sometimes called a Maida regulator on DIY audio. I also re-visited a similar circuit out of The Art of Electronics that used a LDO LT3080. All use a similar approach, a mosfet to drop excess V followed by a floating regulator.

I built three circuits, The AOE one, the 21st Maida and the LM317 HV reg. I'd built the AOE one previously and found it unstable, this was my fault, I left off a 2.2u cap off the OP. This time I found a 10u 450v film cap in my stash, this improved things, but the circuit was still unstable, everytime I tried to adjust it, the OP oscillated. All of these suffered from the same fault too, adjustment resulted in the OP jumping to the lowest V setting, then no I was able to make no more adjustment. All of these circuits were a pain to get working.

So, I write these floating types off, due to their instability and the need for massive brick size expensive low ESR. HV film caps.

Conclusions, for a simple HV reg the 1st circuit and the Sussex version works ok, but they're not 100%. To make a proper HV reg I think a more complex approach is needed, IE opamp foldback which means a separate PSU. For what I need at present this is a bit OTT and I haven't the room. In future I'd definately go down the valve route; valves laugh at HV & HV V reference valves give stable results, a string of zeners drift too much with temp.

Just my inexpert view, Andy.