And she's back again. Just got my Gaming OC back from Overclockers and she's working fine again.
Sorry for wasting your time with that RMA, as I have no clue what caused the system to not POST. But seems I'm in the all clear again.
I'm an idiot. So about the above RMA I performed on my RTX 3080. About a day after I got my perfectly working card again the same issues plagued me. System would randomly shutdown and wouldn't make it pass POST without a few resets/clear CMOS etc. I thought to myself the issue was my motherboard/CPU/RAM (as I'd had a replacement Corsair AX1200i sent to me last August) - Gigabyte Z170X Gaming 7 / I7 6700K. A couple of days ago I received a brand new Asus Rog Strix x570-e / 5800X and a new Corsair Vengeance 32GB 3600Mhz dual channel RAM kit. Tested the board outside of my case worked perfectly fine. Plumbed everything into the case, prayed to Gaben and then no POST. I was gutted. Went to sleep and thought I would have to RMA the PSU as that was the only thing I could think of causing the issue.
Yesterday morning I removed the PSU, disconnected everything and did what I should have damn well done in the first place. Kept everything in the case, and added cables from my PSU externally to the components in the case one at a time. 24pin ATX + 4pin EPS + 8pin EPS first and it booted fine. Left the GPU out of the equation and then started adding the SATA power connectors back in. Failed to POST at that point. Turns out I had a burnt out SATA power connector. I had been using an NZXT internal USB hub since 2016, it's powered by molex but I had been using a molex to SATA power connector to save having to use a string of molex connectors for one connection. All that pain caused by a cheap Chinese connector from Amazon. Removed the NZXT USB hub from my system and replaced the faulty SATA power connector and boom working system.
Moral of story, do a proper test of components on lack of POST (to save money and heart ache!). On the plus side I am happy with my upgrade to Zen 3, so least there was a positive.