looks like an upside down lady with pendulous breasts

Definitely buttocks, Stu - I've been studying it for quite a while now!    Others maintain the figure is of a male, licking his own anus, but on closer examination you can see the tongue entering quite evident, though much eroded, labia majora, which makes this carving quite unique.

I'll PM you with the location so you might check it out for yourself; so much of this stuff is getting vandalised at the moment that locations become guarded secrets!