Thanks for all the info on input protection.
I also work on old valve equipment, so I have made an HV box with a series 630V .22 capacitor, followed by two back-to-back zeners. They should have been 20V, but are currently 10v (serious over-protection!) but they were the only ones in my box of zeners. This HV box is meant to be used always with a 10:probe.
It is obvious, (now that I have found the manual) that one can raise these zeners up to 100V. Any charging current for the capacitor will come through a limiting resistor in the probe of 9M which should clamp the input fairly well.
A further project is to add a switchable 100k to ground at the input of the HV box. Together with the 10:1 probe, this will give 100:1 attenuation, and reduce the Dc voltage to one hundredth of the probe-tip value.