So... here we are!
Inverter powered up and running smoothly
(just a brief initial panicking because, being the multi-turn voltage adjustment trimmer factory preset to almost full travel, on first power up the output voltage jumped abruptly to a worrisome 460V
But after adjustment I got a steady three phase 380V 50Hz featuring a nice sine wave from a 48Vdc input.
Now for the heavy load tests... (as soon as I find some suitable resistor)