Send a message to the person who requested a photo of the grave monument of John Cocking with the following comment:
Richard Cocking was one of my 3XGUncles. My information is that his son James died in South Africa, if this is correct it is unlikely he was buried at Calstock and my guess is that he was commemorated on his parents' grave. Similarly I have so far found no death record in Cornwall for his son John so possibly he died abroad as well. A good quality image could shed light on this.