If you find yourself up around Stannon or Brown Willy there is a cracker on Loudon Hill SX 138804. It is about 8 to 10 ft in lenght and sits at just the right height to get a good grip at one end. It takes a bit to get it going but then slowly it starts to rock and I got a good rhythm going. Amazeing that with very little force you can move something so large.
I have never found the ones down west, I am not sure if the one on Treryn Dinas rocks anymore.
Mr H