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

Hi, I have coped this reference from then OS map. https://explore.osmaps.com/pin?pinCoordinates=50.145131834759724,-5.663548124834676
It’s marked in the map but has no name. Which I thought was unusual for such a prominent stone. It’s covered in Lichen and looks like it’s been there forever. So who ever put it there in the 90’s dis a great job.