There's actually (at least) 2 in Zennor - the Giant's rock, down behind the church and across the fields somewhere, which I think is the one you refer to.
The other I know of is marked on the map at the top of Zennor Hill...
Yes! The Giant's Rock - that's the fella... Went to look for it one day bt go disconcerted at an early stage cos I thought it would be visible from miles around, but could see nowt! I was in a rush... Did see the Mermaid chair thing though.