I have tried to find this before and failed so after a little research I was better prepared. The cave is at the bottom of the larger square rock in the middle of the clitter and in the centre of this photograph.
What I didn't expect was to find a Piskie coming out!
The view across the moor from the entrance to the cave....
...and the zoomed in view to Plymouth Sound from the top of Sheeps Tor with the breakwater and ferris wheel clearly visible