There has been a Whooper Swan with the Mute Swan flock along Deeping High Bank east of Deeping Lakes for a while now so when I saw this bird I thought that was it. Then I noticed the amount of yellow which is more like Bewick's (but still quite extensive). Then I found a Whooper Swan with typical yellow on the bill and a dirty mark on the back of its neck. The picture below combines photos of both birds.
Despite the yellow I think the first is a Whooper. The head and bill shape are right for Whooper and, although it was on its own so difficult to make a direct comparison, it appeared to be the same size as the other Whooper.
Whooper Swans Cygnus cygnus & Mute Swan Cygnus olor
Digiscoped with Canon Powershot A640 and Leica APO77 x20