wgeoff

It's a Scaup KK.

Front of the wing mid-grey colour (black in Tufted Duck)
Back light grey (black in Tufted Duck)
Only a small amount of black on the tip of the bill (more black tip in Tufted Duck)

Good photos of medium rare birds in UK (Red-necked Grebe, Great Northern Diver and Scaup).

wgeoff

I presume it's a Greater Scaup (Arytha marila) KK, this is by far the commoner of the two in UK.

Lesser Scaup is a North American species, a vagrant in UK.

I'm not sure what the difference is but it isn't much.