Could it be that the cromlech is what we know as the Devil's Den and the place marked "Devil's Den" is 'just' a place called Devil's Den?
I know the track may have moved, but it's the 'wrong' side of the place marked "Devil's Den" but looks right in relation to the cromlech (from memory).
And presumably the cromlech could have then become named after the nearby "Devil's Den"....
love
Moth
PS Would the place marked as "Devil's Den" be about where there is now a barn?