Windows 7: Weird erratic boot up..No Boot screen..what have I done!?

Enter BIOS setup to see what Fast Boot (or similar) options are listed, then toggle them to see if they make a difference.

If you haven't already reset BIOS to defaults I'd do that first. What is SATA controller setting?

When you lose the BIOS POST screen, what exactly happens or doesn't that should?

The BIOS POST screen happens first thing, before the OS is loaded. Disabling applications from loading in Windows with msconfig shouldn't have any effect on whether you see the BIOS screen or not. POST stands for Power On Self Test. Basic hardware detection is one of the things done during the POST. If the BIOS is set to quick boot, minimal checks are done. If you turn off Quick Boot, more in depth checks are done. Failing hardware can cause a failed or prolonged POST/BOOT. One of the last things done is that the hard drive is ID'd and checked for an OS. Then command is transfered to the OS.
Don't get me wrong, using msconfig is a good idea when trying to figure out whats screwing with windows. In this case I just think there is more than that going on on your system.
Todays systems can boot from USB devices and I've seen quit a few threads where an external USB device prevented the PC from booting up in a timely matter or at all in some cases. I've also seem some pretty strange things happen when the power supply is flaky.

Testing the 'flakiness' of the PSU probably will prove difficult....this being a self build.

How likely is it that a 'quality' PS like this is going down the pan at this stage???

Difficult to tell if the BIOS is set to a quick boot option....as I can't access the BIOS!!!

New parts can fail just as easily as old parts. I've had stuff that was DOA right out of the box. You can get defects in manufacture that don't show up until the part actually sees some use. All I'm saying is don't rule something out just because it is brand new. If you follow the steps I listed in my earlier post it might get you some answers and maybe let you get into the BIOS. No BIOS POST screen, screams of a hardware problem to me. I'm a retired electronic technician. I was trained in the maintenance and repair of complex electronic equipment, including PCs. I'm not bragging, I'm just giving you some background on what I do and why I come here, thats all.

Appreciate that Alpha (...feel like I know you...!).....will have PSU checked!!!

Just remember that its just a guess at this point as to whats wrong. It could also end up being a motherboard fault, or something else. Even on the best of days with lots of spare parts to swap in, it can end up being hit or miss until you swap the right part, or find a loose cable or wire.

.....finally got around to re-seating video card .....and
.....hey presto!!!!.....looks like all is OK now

Many thanks

OK, hopefully thats all it was. Cheap fix too.