Long-tailed Duck?

Hi folks. I was asked by a friend to go and have a look at a mystery Duck. The nearest I can get to it seems to be a Long-tailed Duck, possibly a juvenile, although I had thought that these were mainly marine Duck's. It's on a small Loch in West Lothian in Central Scotland, and has been there for several days. The Duck is a diving duck and is able to stay submerged for quite a considerable time. I didn't see it flying, but my friend has told me that it a very fast flyer. He also reports that the bird is a bit of a loner, and seems to prefer it's own company, generally avoiding the other Ducks on the Loch. The Loch is a bit out of the way, so it tends to be 'wild' birds that go there, as opposed to birds that are accustomed to being fed by people. Apparently there was a similar Duck appeared on the same Loch for a few weeks at the start of the year and that particular bird stayed for a few weeks before moving off. The photos are really heavily cropped so please excuse the softness about them. I've lightened them up a bit and also tried to add a wee bit of sharpness as the bird was a long way off. Any help on a positive ID would be much appreciated.

Thanks in advance

Paul