Foggy Oxfordshire morning. Blackbird sized bird attempts landing on small pond, gives up and prospects round the edge. I think shorebird! then I think dipper because it bobs like a dipper and its colouration is similar. It has a pure white chest and belly but the rest of it appears a uniform black including -Legs longish, beak longish and narrow. It set off on triangular wings like a miniature Oystercatcher to the SE.
I am probably missing the obvious but it flummoxed me.