Spoke too soon on the Amprobe 2020 paower analyser. A bit more testing with various single phase connections in 3 phase 4 Wire mode ( I don't have a 3 phase supply) showed up some Voltage errors. Blue channel is wrong for starters.
Fortunatly the PCB layout is fairly logical. The inputs are a resistive dividers to a MUX then op-amp buffers. A bit of signal tracing with the trusty Fluke indicates a faulty MUX IC. Tha one on the bottom left. It's a 74HC4052 which I don't have in stock. I've a CD4052 in DIL but it's not worth hacking it in, not with mains floating around.
So off to order a 74HCT4052M. Or 3. Plus a couple of MAX 492 and 494. Even if I had to replace all the MUX and buffer IC's it would be worth while.
Interestingly the PCBs are marked HT instruments not Amprobe.