I did some more testing, when disconnected the PSU itself seemed to work, output the 5v/12v, and when the power board with the 3 LEDs was connected it seemed to work ok, with all LEDs lit up. However, once the power board was connected back to the backplane it all went dead, so I guess it was unable to maintain the current. I also took the opportunity to pull out all the cards for a visual inspection, and everything looked normal, no obviously damaged capacitors, although I guess it's possible for some to have failed without being visually evident. Probably needs a deeper dive.

Anyway, I have now ordered and fitted a new replacement XP power unit, ECM60UT31 and fitted that, and now it's all powered up just fine.

Sources: 800D, Sooloos Control:15, Sony BDP-S790,Humax Freesat+
Control: 861v4, HD621
Front: DSP3100
Centre: DSP3100HC
Rear: DSP3100
Screen: Samsung UE50H6400
Projector: MF-10