At WWT Slimbridge the other day there was a largish population of tufted ducks. These ducks are pretty common in the UK, easily identified by the characteristic white side and the tuft at the back of the head. The female is much less distinctive, so is less easy to identify (see above link).

The pale blue-grey beak adds an attractive touch of colour to the otherwise essentially monochrome male. In the right light, there is also a suggestion of green on the head. You can see this and the tuft better in the following.