Moved paragraph 3 to the top, to cut a long story short - my PSU is affected by temperature, it musta been about 4 degrees C in the room I had my PC in, heating it up with a fan heater for 5min would've saved me 2.5 hours of troubleshooting. I tried it as a last resort out of desperation, and because I knew from studying physics that heat affects electromagnetic induction and static electricity, so it wasn't totally mad, but I still found it mad that it worked first time like there never was any power/PSU issue... if anyone says they have an LED blink followed by nothing, ask them the temperature of the room it's in, don't just tell them their PSU is borked, even though it might be, as PSUs do break on occasion. My main point is that temperature in cold places/at cold times of the year should be part of the troubleshooting manual for static/EM induction/twitchy PSUs I hope someone spreads the word in troubleshooting circles for the exact problem I had and saves other people thousands of man hours, even if it is troubleshooting step 11 after the other 10 steps.
I just spent 2 and a half hours trying to figure out a quick flash from MB and DVD drive LEDs, followed by silence and no second flash even if I tried tapping the power button again, my setup doesn't even have a PSU LED. This is totally irrelevant to your situation and I realise failed motherboards do cause this and have seen such written on other threads regarding new PC builds, but I wanted to share the solution in an active thread, with hopes people will copy/paste it or just remember it for other people who might have the exact same problem I had.
I tried all the normal troubleshooting, re-housing pieces, checking for shorts/static, CMOS etc. and was stumped as to what the problem was, suspecting PSU failure, this isn't the first time it's happened with the same PSU and it has come back to life eventually, I thought in previous successes that fiddling with the power switch lead on the MB solved it, but despite trying that many times, it failed to resolve the issue this time.