Spongeware...

...apparently. Dug this up recently at the allotment and D's esteemed colleague Big J identifies it as "spongeware" from 18th or early 19th c - only reference I could find online was that it's being made again these days in a pottery in Bennetsbridge, Ireland.

Had to fiddle a lot with the hue to give even a slight impression of the strange gray-green colour.