Should anyone look to this thread for a resolution to a totally blank screen look first to your case! After many hours soul-searching I finally noticed a PCI card had become very slightly adrift causing the chip's intitial checks to immediately halt the boot. No beep I hear you cry - well, no there was not and as the warning beeps function perfectly well I can only assume the boot process had not got that far! But why the case? Well, the thing is so flimsy at the back when I moved the tower it wrenched out the card!
I just had to share my misery.