Visited 15th April 2003: When I visited this site I thought it was Carn Ferched, but now I'm more inclined to think it's actually a standing stone called Maenllwyd y Rhos.
The site is relatively easy to find if you're on the ridgeway path that runs along the Preseli range. There's a large boulder by the footpath opposite the cairn, so if you're not sure look out for this, then head north and you'll get to it. It's clearly visible from the footpath, so this technique should only be necessary in poor weather conditions.
I was surprised to find such large kerb stones in what I thought be a cairn, so I checked and double checked that I was in the right place. At the time I was suspicious of my own navigation because the Landranger map, which marks Carn Ferched, puts it further west than this site. I also had some grid references for Carn Ferched which I'd put in my GPS from Prehistoric Preseli by N.P. Figgis which didn't match the site I was looking at.
It's only a month later that I've realised that these stones are probably not Carn Ferched at all. My grid references exactly match a Cambria Archaeology record for Maenllwyd y Rhos (SN 15441 32906), which is listed as a possible standing stone. What little information I can find on Carn Ferched suggests it is a barrow like site, which certainly doesn't describe the stones that I saw.
Whatever it actually is, I visited this site twice, once on the way to Bedd Arthur and once on our return. Same day, but very different light conditions. Well worth the tiny detour if you're heading this way to Bedd Arthur, Carn Meini etc. Possibly a bit trickier to find coming from the direction of Moel Drygarn.