Hi Hetty,
What scale OS map are you looking at? I can't see anything on Trewellard Common on the 1:25000 (Explorer). I know there's a modern standing stone up on Carn Eanes somewhere, Craig Weatherhill posted something about it on here years ago to say that it had been put up in the 1990s I think. Might that be the one you're talking about? Apologies I'm not much use with latitude and longitude, I much prefer OS grid references!
tsc