After a good look through ID guides and at online material, I agree with Dave that this bird is probably an adult male American wigeon, though I have to admit that duck ID is not a specialist subject of mine. Initially, the background colour of the cheeks seemed to lack the cold, grey/white tones that are characteristic of American wigeon, and at some angles in a low sun they appeared quite rufous, but they are still within the range of variation shown by American wigeon. The most helpful article was by Steve Votier et al in British Birds Jan 2003, link here. In particular, Plate 18 of that article, shows a similar bird among a variable range of others. The black line along the base of the upper mandible is supposed to be a diagnostic feature, and this shows on some of the photos. Whilst watching the bird on Thursday, Rich D was on the phone giving us many helpful pointers including the fact that pure white axillary coverts would be diagnostic for American wigeon but we didn't get a chance to check out this feature. It was disturbed several times by someone searching for a good spot to dig bait, but it was still almost impossible to gain an impression of the underwing colour. Hopefully it will stand up and flap in front of camera one day.