Someone donated this UPS and I was fully expecting the SLA batteries to be
sulphated and scrap, but they were in fact reasonably charged and a session
on the Optimate gave each battery the green light "good indicator".

With the batteries fully charged and the mains connected the UPS remains
dead as a dodo - no LEDs lit, nothing!

A look on the website reveals that the unit can be shut down via software (I
don't have the serial cable) but I would have expected at least 1 LED lit on
the front panel, I've opened it up and confirmed both mains & battery
voltage actually reach the PCB.

Any suggestions?

TIA.