Please call the RSPCA and report this immediately. They will try to rescue it and get it help. A predator could get to the swan or it could be injured and need a veterinarian. If you leave it overnight, it might not make it through the night. If you are close to Eton, the Swan Lifeline should be called and they will help. It seems that this swan got into someone else's territory. So, if it is rescued, they will try to find another area to release it. The Regal Swan