"It took a while to get my head round it but looks like the 555 timer is used to send a stream of pulses into a darlington pair of transistors that drive transformer T1, which drives the BU460D transistor which chops the LOPT primary coil in and out of circuit, I guess constantly collapsing and reforming magnetic field in the LOPT secondary coil gets dumped to the tube up the annode wire to the cap. The tube itself is a giant capacitor so it handles the smoothing itself. The duty cycle of these pulses is what sets the resulting voltage. The HV shutdown seems to be controlled by a comparitor circuit, with a voltage divider using the +24v as a reference voltage, kicking a thyristor in the circuit to bypass a small capacitor in the 555 timer circuit when a threshold is breached. With the capacitor bypassed the 555 oscillation stops and the HV is cut."
Well duh, of course!
Just messing, superb work, you should be very proud.
Amazingly detailed write up, thanks for taking us on the journey :D