Bradford Pool is also (or was) known as Bradmere Pool. Mere (Middle English, from Old English) is generally a small lake, pond or especially a marsh. There does not appear to be a stream feeding the pond, but the soil here overlays a bed of clay, and water appears out of the ground as if by magic when there is the least bit of moisture about so I'd hazard a guess that there might have been a marsh before the tinners made a great hole. I think this pretty well fits in with what Tom says - probably a Broad Marsh