Trying to identify a wading bird

I've seen these birds flocking together in the Dee Estuary at West Kirby, Wirral, and would love to be able to identify them. I've scoured the RSPB web site and considered Redshanks, Dunlins, Sandpipers, Sanderlings and Knots, but all seem to have something not quite right - too long a bill, wrong colour of legs or plumage, ..... I'd be grateful if somebody could help. The picture was taken last December, so presumably they have winter plumage. Regards. David