Have I screwed my new build?

Does your motherboard manual tell you what the flash means? Is the LED on your motherboard or your case?

Faulty PSU probably. Grab another and test.

does swapping ram do any help?

keep your head clean, take a deep breath and think about every single step.
Is your power supply for Motherboard and Processor installed correctly?
Uninstall every hardware you dont need. HDDs, CD, PCI-Cards, Graphicscard and so on.
Only use one RAM not all.
Still doesent work?

Check 20(or 24) pin and 4 pin entries on Motherboard.

You have this problem because your Motherboard doesn't get any power to operate.It's either your Motherboard or PSU, check both of their connections.

Also OP, check your power/reset connector headers are inserted correctly. Could explain why you see a brief flash of light.

I remember a friend having the same problem. Turns out he didn't use motherboard standoffs and was shorting everything :D Lots of fried hardware...

Check the back of your PSU and see if it is set to the correct voltage for your country. And if it has any beeping sounds it will be easier to diagnose the problem.

Bump anyone D:

sounds like a power supply problem, but i may be wrong

Check your compatibility, check to see if all your connections a connected correctly. If nothing else take it to your local electronic store and let them have a look at it.

I hate to say it but there's a small chance of a faulty hardware when building, if you have other spare parts to swap in and check that would help.

Just wondering... Are you sure that your PSU is enough to power your entire system? A friend of mine had the same problem and that was the fault.

Check that RAM is correct type and properly seated.
Check that CPU is supported by Mobo and properly seated.
Check all Mobo power connections and that they are properly seated.
Check if discrete video card is supported and properly seated as well as power connections being connected properly.
EDIT: Also check that you have connected front panel (start, reset, etc ...) wiring correctly.

Check the screws under motherboard. 1 too much and it wont tunrn on.

Check the screws under motherboard. 1 too much and it wont tunrn on.

Had same problem with an older PC. MOBO had broken RAM slot and PC just didn't turn on. Removed my RAM stick by stick and found broken one. Anyway - I suggest you to ask professional to check your computer. And another thing - did you turn on your PSU? Because if it's turned on there won't be anything else than a single flash on a LED light near turn on button. I know it sounds stupid but might happen.