I tend to agree with Moth, the Cromlech looks right in terms of distance from the road and the fact it's on the left (no sign of a fork in the track though). Maybe the Devil's Den was a copse or field name (unlikely a barn would have such a name). BUT, how come Rhiannon's folklore about the Devil being connected with the actual stones, rather than a wood etc?
A bit exciting though isn't it. Worth a quick walk up there this afternoon by a conveniently placed local, don't you think?