Here is the stone Rupert is talking about...
http://www.themodernantiquarian.com/post/65688/bryn_celli_ddu.html
but there seems to be no more info about what it is. Surely you would have to take some stone off for investigation to prove one way or the other, which of course means defacing it.
Coflein gives it as 'tomb guardian' phallic symbol stone, not sure it is the same one but fascinating none the less......
http://www.coflein.gov.uk/en/site/93827/details/BRYN+CELLI+DDU+CHAMBERED+TOMB/