It’s K101.
After tracing the fault, I confirm that on pin 4 the incoming DC signal looks good, on pin 3 when the unit is reading erratically I get a suitably erratic reading.
Searching around I discovered a number of discussions about K101, including talk of reflowing the contacts, since I cant get a new relay, I tried that.
After doing this I could see that it made an immediate change, the DC measurement had been a bit noisy and after reflowing it looked a lot better, so I left it in MIN/MAX mode for a day and no more erratic readings.
After another 12 hours it started reading erratic again, some times reading ok, other times reading erratically.
When it’s in erratic mode I can’t short 3&4 and the reading goes normal, so it’s definitely K101.
Not sure where to go from here since I can’t purchase a new replacement part as they’re not available.
There’s some talk of other lower rated replacements, but the 500V replacement isn’t available any more and only a 200V part: