If using a 4k display, try using a lower resolution one. It's possible you can get some image

Try reset the cmos ?

Appologies, I've forgotten to mention my efforts with that recovery procedure.

I tried using a lower resolution monitor via HDMI, no change.

I wondered if my GPU wasn't supported so I found an older GPU. Connected the monitor via VGA, no change.

I had the same. I have a 4K LG monitor and a NVIDIA Quadro 4000. 

I changed my monitor aspect ratio to “original” and plugged the Display port cable into slot 1 on the card. It worked. I’m not sure if this had anything to do with it. 

I removed all other drives to use BIOS recovery on USB stick. 

The BIOS recovery process using a USB stick didn't work for me. I guess my BIOS was too far gone for the recovery process to work, or perhaps something else was faulty.

I never did manage to fix my PC. Dell refused to look at the unit and wouldn't even sell me a replacement motherboard. Their reason was because the previous owner of the unit was given a replacement unit under warranty, the unit should have been returned to them. Somehow, it's found itself on the secondhand market which is where I purchased it.

Thankfully the supplier I purchased the unit from, even though it was out of their warranty, replaced the motherboard and shipped the unit back to me free of charge.

So, I did get a resolution, of sorts, thanks to the good will of the supplier.

"I removed all other drives to use BIOS recovery on USB stick. "

Did that solve your issue? How did you prep your USB stick?