In reply to Ollie B:
Hi Ollie
I don't know the details of MCN's signage, but I do know the area very well. Most of the route is on well established paths, tracks or roads, though the landscape looks very different in some areas because millions trees in South Wales have been felled in the last few years, and those areas of forestry are still marked on the map.
I'd say the most fiddly bits in terms of nav are:
Where the extra loop starts, but there's a checkpoint there so presumably they'll direct you and signage should be good.
Around checkpoint 2 - this looks horrible!
The Bog of Doom after checkpoint 4 - largely because it is indeed hellishly boggy rather than because the nav's particularly difficult.
The ins and outs of the bits of road and PROWs navigating around the northern escarpment.
So those are the bits possibly worth having a closer look at, either in person or just by studying the map. The Where's the Path website is very useful for that, though the map is 1:50 rather than 1:25.
http://wtp2.appspot.com/wheresthepath.htm