Specs - i5 3570k, msi gtx670, 2 x4gb corsair vengeance lp 1600, corsair h80, samsung 630 256gb ssd, Sony 5850h bd-re. Silverstone sst-1200G psu with the PP05 short cable kit.
I have already tried one ram stick in the furthest slot, but still no post. May have got a different q code though.

Finally had a chance to try some of the suggestions. I have replaced the 12v CPU connector; tried each of my RAM sticks (2x 4gb corsair vengeance LP 1600mhz) in the far red slot alone; replaced the graphics card with an old 7800gtx that I know works and tried that in each of the gpu slots; as well as clearing cmos several times. Still no post.

In terms of Q-codes again a variety - 62 and 63 mostly, but 72, 78 and 98 a few times and also d0 and b6 a couple of times each.

In terms of the Q-LEDs the VGA one stays on most times but not every time. I don't think it is a problem with the graphics card as I have used my old one which works.

I haven't tried reseating the cpu as I would need to dismantle the cooler which was a pain to fit and haven't had time to do that yet - but will do so if you think I should.

I note that it being a RAM issue seems to be the consensus - so will probably rma it.
Any other suggestions?

I just had a similar problem.. I changed the 3rd set of timings (as per the OC guide in terms of stability) and all is well. See this thread: http://rog.asus.com/forum/showthread.php?21693-Maximus-V-Memory-problems&country=&status=

I just bought my motherboard a few days ago and it was working fine the first 48 hours and it randomly shut down on me last night and now I'm experiencing the same symptoms as you. From reading mix reviews on Amazon and Newegg it seems that these models are having problems to POST for some reason, after trouble shooting I decided to call ASUS customer support and they advised me to RMA after testing over the phone. I'm glad Amazon has a great return service and will be getting my new motherboard tomorrow hope everything works out for you.