The sinensis Cormorant (the continental subspecies of our local Cormorant) was still considered a national rarity in Scotland until earlier this year. There have now been over 20 records of this subtle subspecies, most of which were seen in Lothian, including this bird that I found at Musselburgh in 2010. As a one-time description species I think it is now merely a notable subspecies at local level, though I don't think I'll get out of the habit of looking for them whenever Cormorants are near enough for scrutiny.
The identification hinges around the angle that the bare skin forms relative to the cutting edge of the mandibles. There are lots of diagrams on google, but this is a good one. In today's bird the angle was easily 90° which puts it safely in the sinensis camp. I think, though I am not an expert, that the bird can be aged as a fresh juvenile by the brownish plumage, pale tips to the coverts and generally unworn plumage.