Hi Robin,
I've been using the RBM EHT meter getting on for forty-five years.
I'm sure the load that the meter imposes on a mains derived EHT system is negligible. The source impedance of EHT from a mains transformer is far less than any flyback EHT system, be it colour or black and white TVs.
Electrostatic meter explained:
https://en.wikipedia.org/wiki/Electrostatic_voltmeter
DFWB.