Well out of the area today, I was over on the Wirral at a hotel.  In the grounds, I came across a pond, perhaps a few tens of metres long and maybe 20 metres across.  I was surprised to see what looked like 2 wild swans, together with a few dozen mallard and a couple of Moorhens.  Although the place is probably only a couple of miles, as the swan flies, from Burton Marsh, I was still surprised that there could be wild swans in such a small area of water.  I managed to get a few poor quality pics (only iPad to hand, unfortunately) and have attached one of these.  I'd value thoughts as to whether these really could be wild swans and, if so, Bewicks or Whoopers (I'm not skilled enough to tell bills apart).

Many thanks for your response, David.  I have run various web searches and have not found anything on feral Whoppers on the Wirral - I will endeavour to find out more from birders in that part of the world.