Having thought about this further, I think that I will go with the following (attached) circuit. To explain how it works. The 10M resistor R1 will bring the dc voltage at the probe tip down to just over half its value, removing some of the stress on the blocking capacitors in the box, and in the 320...

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...