Elite 8300 has red flashing power led and 4 beeps with no POST

[Clemaux]

Just replaced PSU with a Corsair 700W modXstream-pro because my Gigabyte nVidia GEForce GTX 750 TI is one of the rare GTX 750 cards that absolutely need a power supply by way of a 6pin socket on the top of the card.  My previous card (Radeon HD 6450) went wrong and I got nothing on my monitor at all.  Once I removed the Radeon card I got my POST beep confirming to me (at least) my graphics card (or, gulp, my graphics card slot in my motherboard) was the problem.

Replacing the PSU required me purchasing a power cable (AXT 24pin to motherboard 2-port 6pin adapter power supply cable for HP Z220 &  I NO Elite 8xxx) which came with a  a power converter for the purple cable reducing 12v  to 5v (or vice versa!).   It seemed very easy actually despite my initial trepidation and the modular cable system made powering the new GTX 750 TI easy enough  for a toddler (or even me) to do!

Feeling confident my problems were now resolved I switched on.  Ugh, nothing on monitor at all, no POST completion but a sickening red flashing power LED above my PC's power button with 4 beeps going off all the time.  Depressing the power button (and keeping it depressed) gets both my power and HDD light green with no beeps but as soon as I release the power button I get back my 4 beeps and red lashing power light (with no HDD light).

I have no idea what is going to on now or what I should do (other than to chuck this blasted thing into the rubbish and go and buy a new one - or maybe even build my own since I have quite a few components already including a new 250GB SDD, a new 500GB HDD, my PSU and my graphics card!!!!).

Can anyone throw me any sort of a life line?