Dell PowerEdge 2600 won't boot

I have a Dell PowerEdge 2600 server that I have purchased recently. I bought it for parts as I needed a replacement power supply for a client. I saw the server boot up when I bought it but the guy did struggle to get it to boot initially. It did what it's doing now.

The seller said that it had done this a few times whilst he had it but he put this down to the fact that he'd removed the hard disks. I don't buy this argument though.

I've been carrying this server around in the back of the car for a week or so and I've since obtained a replacement power supply for it to replace the one I used for my client. Trouble is, it's not booting. It sits there with the power light (the one in the middle of the button) steadily flashing green. There are some fans running but not all of them. Nothing happens when I press the power button.

What I've tried:

1. Re-seated memory
2. Re-seated processor
3. Re-seated power distribution connectors
4. Removed power, battery pack, CMOS button battery and pressed the surface mount 'reset' switch.
5. Obtained a second replacement power supply
6. Removed all non-essential cards ( a second NIC and a USB expansion card)
7. Re-seated all obvious cards that appear to be re-seatable.
8. Tried installing a hard disk

There are a series of LEDs on the motherboard for diagnostic purposes. All of these are OFF but there is an LED close to these which is on.

On disconnection of power, the rear cooling fans tend to spin a bit which demonstrates that power is getting that far.

Initially, I thought maybe I had a faulty power supply but I now have a second replacement and I'm getting the same results.

Any ideas?