Well Well! I feel better knowing that I was not the only one in two minds with this bird.
Most of the swans seen today, out of close on a 1000, were Whooper's. There were a few small groups of Bewick's dotted about, and were fairly easy to see as Bewick's, when in small groups on the water, just by their more dainty manner and more compact sizes. Also, when on the ground, it was fairly easy to look at a group of 'bewicks' then scan over a group of 'whooper's' and compare. The one posted, was flying on it's own, and, as I said, it did look shorter in the neck, but I don't mind admitting that I found trying to decide a solo bird like that, as Whooper or Bewick's, very very difficult, size etc, isn't easy to determine when birds are flying.
With the bird in question, I favoured Bewick's based purely on the shorter neck.