Hi John,
the possibility of Scaup did cross my mind, but the photo was taken near me at South Norwood, nowhere near the coast. Here's a link to a google earth placemark:
http://www.birds.deansfamily.com/kmz...ood%20Park.kmz
The bird in question was also with some Tufted Ducks. It's on the right in the background in the first photo. The second photo is a higher-res close-up of the original shot.
Thanks, Simon.