It will make more sense if you watch part 1 of this video first, Link to part 1 below.

Link: http://www.youtube.com/watch?v=OthR5H6WW_o

Basically I got the following embarrassingly technically illiterate reply from a member of, I assume, ebuyer's technical support team, regarding the reason why a Casecom 500W PSU will not power up:

"Have you tried using it on a lower power system? The power supply you show
replacing it in your video is considerably more expensive and also provides
a continuous flow at 430w rather then a peak flow at 500w as per the
Casecom spec. Each power supply is advertised correctly with regards to
their respective wattage.

If this is the case and you have selected an under powered PSU for your
system or an unsuitable PSU, we will be unable to process this return any
further as compatibility is solely the end users responsibility."

So in the video I will demonstrate that a 350W Casecom PSU will happy power the system up and that the reason the 500W one will not it not because it does not have enough power, but because it is FAULTY and anyone that knows anything about IT hardware would know that.