Thanks heaps for the info so far

All my points are on a RCD, and am very particular about unplugging.

Dunno if another angle will help at all

The black PSU is                          silver PSU

 +12                                          +5
 -5                                            GND
 GND                                          GND
+5                                             +12
+5                                             NC
FG                                             FG
AC                                            ~
AC                                            ~

GND's are connected
+5's are connected
FG's are connected
AC's to ~ are connected

there is no connection between the +12, and this would be due to the fact that there is no +12 on the black PSU - spot on Firepower - it seems the silver one is an add on.

When i took the FG off the black PSU there was no reading for resistance (or continuity) between FG and GND - same on the silver one.  Resistance between the FG and the earth screws was around 0.5 ohm