I've not thrown the old PSU away, it's behind me, so I could put it back in if nothing else works. The PSU is 4 years old now and apart from the noisy fan, it was fine. So no warranty left.
So the issue I'm getting is this, I turn the monitor on and press the power button on my case. Lights come on, fans spin but nothing happens on screen. After about 30-60 seconds I see the Gigabyte post screen and everything loads up as normal.
I've checked the drive status. Everything is fine and in good health. I'm typing this on my pc after just being on Rocket League where I saw no performance dip or anything. It's just that initial few seconds after pressing the power button that seems to be the issue. Oh, and I've also flashed the latest BIOS onto the motherboard, just in case that helped, it didn't.