Ah, it was you hob, with whom I corresponded last year about this...

I was there again in July, and took this shot down onto Caer Bach ascending Tal y Fan on the way to Maen Penddu.
The white stone is clearly visible centre left - and is not a sheep!
http://www.megalithic.co.uk/modules.php?op=modload&name=a312&file=index&do=showpic&pid=8794

I am there again on Friday morning - probably arriving at the location by 8am! Will take another look - but I'm no geologist!

Cheers

Tim