Hi Colin,
Both Bing and Maxar imagery suggest the water feature is enlarging - my guess from Northumberland experience is mining subsidence is lowering the surface level. 
As you mention the size may be changing, I've added intermittent=yes.
I've also added some more detail to the adjoining area, but crop rotation suggests the landuse=meadow/farmland may be wrong (imagery shows sheep, but they could be clearing crop remains).
As ground truth always beats armchair mapping, a review would be appreciated.
https://www.openstreetmap.org/changeset/98443117
Happy Mapping,

James