G5 intriguing boot issue

Hello there,
I have a strange set of boot issues with my G5.
It started playing up yesterday, failing to boot. To begin with the problem was intermittent, then I started to get a grey screen with repeating text, and now it won't boot at all.
I've tried booting from my backup drive, but the G5 ignores the keyboard combination to force this. I've tried holding down the mouse on boot up to open the DVD drive, but that doesn't work either, so I can't boot up from a DVD.
Initially I got the boot up "bong" but now that has stopped as well. The little blinking "on" light blinks a couple of times, then pause, then blinks some more. If I leave the computer after failing to boot, about 3 or 4 mins later the fans start to slowly rev up into overdrive.
I've tried swapping out the RAM, I've reset the SMU, and replaced the internal battery - none of them have worked.
Nothing has been added recently by way of software or hardware. And I've removed all peripherals.
Any suggestions? Is this hardware or software?
Thanks for your help.
Jonathan