Knot or not?

Please can someone help identify this flock? I saw them wheeling about by the river Ore at Orford - then land on a field on the marshes.

I have been looking at various bird books but am a bit confused.

Also, can anyone tell me how to load my pictures up so they appear full width, rather than half the size of other people's posts? thanks

They appear to have grey legs and short beaks, but a black tail in flight.