I was able to park really close and went through just one gate, but still went the wrong way and then saw it on the way back to the car, d'oh.
Not enough room under the capstone for a person but enough for a camera .The capstone is maybe three metres long and propped up on smaller boulders at one end whilst the other end creeps under the grass. The whole thing sits at the eastern end of a mound about thirty feet long.
Visible from the B4401 between Llandrillo and Cynwyd. You need to jump a couple of gates and avoid the chickens to see it closely (had there been a farmer I would have asked). One of those sites where you're not fully convinced you've actually found the genuine article and not a random capstone-like piece of rock.