Right,
Basically I was preparing to have another go, rebooted to Safe mode for DDU and noticed that the display on the GTX1060 I swapped in was glitching a bit, then the entire thing dropped out. Then I found that I couldn't use my iGPU anymore either despite deliberately having the dual display setting enabled as I planned to go into Windows to see if the 4080S was detected. Anyways - black screen issues(!) Tried a boot USB and nada. After several cycles of GPUs and finding none of them working properly anymore, I assumed the motherboard was kaput or something was loose. I honestly thought my PC was busted for good - at one point it wasn't even POSTing.
I pulled absolutely everything out bar the CPU and re-seated it. POSTing but no display. Then I did a full CMOS reset pulling power cord, pulling the battery, holding power button for 10 seconds, shorting the reset pins etc. Finally got it to POST properly and show display again on my old GTX970. I did a restore optimal defaults and didn't restore a single one of the previous settings. Booted OK (seems CSM must be default). Then I put the 4080S in and I had display so did the uninstall and install new drivers.
I thought perhaps XMP profile was an issue with the 4080S but I've just set that back as it was and other stuff seems much the same as before. I'll have to go through it all and see if it was some creeping death setting I've overlooked or a ghost in the machine that was exorcised with the rejigging and purging.
When in doubt, start from scratch, I suppose. Just ran a quick bench and it seems OK but I'll do more tomorrow. The main issue I now have is that my AIO loop is sitting on top of the power adapter due to the case panel, I think I'm going to have to find something to use as an insulating spacer to avoid heat transfer.
Thanks a million for all your input everyone... fingers crossed
EDIT: I keep typing 3080, for some reason my brain won't register that I have a 4080!
EDIT2: The oddness has freaked me out a bit so I'll probably get a new CPU/Mobo/RAM reasonably soon. 90 Degree adapter for the 12VHPWR thing might be an idea too - hopefully won't melt on a 4080S!
EDIT3: Installing the bracket on this card (MSI Gaming X Slim), it's good to have the case on its side and get some electrical tape to hold the blanking plates in place so you can get the screws in without the plates falling in towards the motherboard.