I called Ailz over.
"See that figure in the central lunette. I think it's a Sheela-na-gig. What do you think?" She thought it was too. But it was a long way off the ground and we couldn't be sure of the details.
When I got home I blew up the image and it became rather more mysterious.
I think we can agree that it's an exhibitionist- that's to say- a grotesque figure displaying "those parts that men delight to see". They can be either male or female- and in this instance someone has taken a hammer to the crucial area so we can't be sure. I imagine the vandalism was deliberate- and probably took place in the 19th century- when people were more irked by public indecency than at any time before or since. The face is well-weathered but I rather think I can see a big moustache and a little beard, so it's probably male. But the really odd thing is it has a large fish in either hand.
Is it a merman? A river god?
Rochester sits on the banks of the river Medway so that last suggestion is the one I'm going for...