Mystery duck

[Richard Chambers]

This post refers to a sighting at Mallard Pike Lake, in Gloucestershire,
(England), in the Forest of Dean, about 15km south of Ross on Wye. This is
just a short flight across the Severn Estuary from Slimbridge Wildfowl
Centre. What I have seen is not (to the best of my belief) a native Brtish
duck, and may be an escape from Slimbridge.

The ducks (there were half a dozen of them) were all about the same size
as a Teal. They all seemed to be female, with no male present. They were
not Garganeys.

Full description:-
Swimming on the surface, and never diving. Size of Teal. Greyish brown (or
brownish grey) head, with visible eye-stripe, light buff. Occasionally,
the head feathers seemed to form a tuft at the back of the head, but on
other occasions the tuft seemed to be held flat. Bill smallish, a mixture
of greyish-pink and pure grey. Darkish brown wings and back. Lighter brown
belly, mottled with darker brown feathers. Dark greenish-blue speculum,
only occasionally visible while the duck was swimming. Pure white under
the tail, clearly visble while the duck was swimming.

Could you please identify this duck for me?

Richard Chambers Leeds UK.