Mystery Duck from Tern Hide

Can anyone help identify a mystery duck seen from the Tern Hide on Sunday afternoon ?Unfortunately no photo (just out of range of my i-phone!)

Size - similar to a gadwall (was in a group of them) . Overall impression low and long in the water.  Short bill. Green head. Yellow flash behind eye . Hint of white above bill. Main body colour - grey/dark grey. But white  at base of tail, and tail rear end was darker - almost black. .

Ornamental or hybrid??

Andrew

(ps nice sighting of a group  of 7 snipe coming out of the reeds to join the green sandpiper. Made up for the wet conditions and the truant kingfisher. Hope the barn owl spotters had luck later in the day) )