Interesting discussion! I have just seen these two swans for the first time today. I can't claim any good experience with juveniles of either species, having never properly studied a juvenile without accompanying adults. It's embarrassing!
I found size and shape very difficult to judge, even after watching them for an hour. They never looked particularly long-necked though and they
are small compared to the Mute's!
As has been mentioned, the bill pattern of these swans fits Whooper very well. However the literature I can find isn't very encouraging regarding whether bill pattern can be used to identify juveniles. BWP is fairly non-commital and "Wildfowl" doesn't help much either.
So pictures like Sean Nixon's on the Surfbirds Scarce Gallery (referred to below; from Westleton in December 2008) are quite interesting. Most of the young Bewick's in the Surfbirds gallery have bill patterns which resemble adult Bewick's. But assuming the Winterton juvs are Bewick's Swans, then their bills look similar to the Seaton Pond birds.
[For another example see here:
http://www.arkive.org/bewicks-swan/cygnus-columbianus-bewickii/image-A16128.html ]
However there is a subtle difference in the pattern of pink on the upper mandible. In the Westleton picture, both birds have pink which cuts across the bill quite a bit above the nostril (ie between the nostril and the eye) whereas the seaton birds both have the pink cutting through the nostril (I've attached a dodgy field sketch but this is also shown very well on Ian F's first photo).
If the pink on these juvenile swans' bills turns black, then the Westleton birds will end up with a Bewick's' pattern and the Seaton Pond birds will end up with a Whoopers' pattern.
So, is this a reliable feature in juvenile swans? If yes, then they must be small Whooper Swans.
I have noticed while looking at all many swan photos on the web, that Bewick's tend to show more of a bulge below the base of the bill than Whoopers, which helps to make the bill look shorter and deeper-based. They also seem to have a slightly different shape of the feathers at the gape of the bill. I don't know if either of these are consistent features but the Seaton Pond birds look more like Whoopers in both respects.