Back up often
Both UPS channels went offline in a cascade failure due to loading. Then the transfer to mains was disruptive and the transfer back to UPS failed.
And the second attempt to switch back has also failed and that involved switching it off and on again so it was a proper IT fix, not some bodge.
Currently it's running on utility power. It's not the first time the UPS systems at Sovereign House have gone out like this either...