An update:
I have now tried with a completely new PSU (I was running on 850W but changed to a 650W, with new internal cables too) and the problem still percists.
I even have a video of some of the strange behaviour here:
View: https://imgur.com/gallery/EJNHdlQ
Thank you for the reply RARRAF, but turns out that my sneaking suspicion that it wouldn't work, was true.
I hadn't tried DDU before, but I have tried several different method of uninstalling and reinstalling gpu drivers. (as I said in my post)
Tried your suggestion and, as soon as the pc had...
(Sorry, this is a longer story, but all in the name of being thorough)
Dear wisewomen and men, I have a headscratcher which, so far, I haven't been able to crack.
TL;DR - My screen is flickering, with random graphical artifacts (lines, colours, dots, icons) on screen, sudden restarts, crashing...