We visited this on our way home from Boscastle.
Access is via the Arthurian Centre, and there is an entrance fee.
I think there is a public footpath you can take. I don’t mind paying to see things (as most landowners have to pay public liability insurance, etc), but that just detracts from what is essentially a wonderful artefact in an attractive setting. Or it would be if it weren’t for the awful fairy-tale pictures of Arthur, etc scattered around the place blahblahblah... and the limited access due to a viewing platform (which is probably a good idea, safety wise).
That said – it’s a cracking inscribed stone! I might see if there’s another route to it on the other side of the river...