Adding a 100k resistor worked. The problem seemed to be only with one channel but caused the effect in both when they were connected to the same power supply (!?).
Latching was to B+ on both outputs, permanently, immediately after the high voltage was applied. I use a cpc1117N as a delay retrofitted to the GRHV boards that didn't have it built in, so the tubes are pre-heated. I've attached a picture below. I'll test more thoroughly but probably not till the end of the week.
Happy to *try* to answer any questions: though I've built quite a few different KG amps and can follow instructions, I wouldn't say that I *understand* anything.