For yet more speculation on the Rochester figure take a look at this page in the Sheela na gig Project's website where it's compared with carvings of jugglers elsewhere in Rochester cathedral and one of a mermaid holding two fish in a church in France. These guys are the experts and- basically- they don't know any more than I do. They do however confirm my sister's observation that the figure is sticking its tongue out.
Their picture is a little clearer than mine. Bastards!