Marty,

you may be onto something here - dunno about my logic BUT

When i test on the PSU (the silver one - is the black one a PSU as well????) there is a good 5V and 12V on the appropriate lugs IF i put the -ve lead of the multimeter on the GND tab of the PSU

However, 

If i put the -ve lead on some of the earth screws (green wires in the pic) i get only millivolts - does this mean there are some potentially bad earths and they all need a good clean or am i wandering down the wrong path all together and i shouldn't be getting a reading off these screws anyway?