ajohn

This is a female Eurasian Teal. Note that the speculum in the wing is glossy green (would be white in a gadwall), and also the white side to the tail (which is a very useful feature of female teal).
The bill pattern is vaguely similar between the two, with orange at the sides of a dark bill, but notice that on this bird the orange is at the base only, while on gawall it would be brighter and broader along the whole of the bill.