There is a lot of information on this site of The Swan Sanctuary - in the FAQ's section the following advice is given:
"There's a swan's nest in a really vulnerable location - what can be done?
If the nest is vulnerable to interference from human factors, such as on a tow-path or the bank of a pond where people walk their dogs, then you should contact your local council and ask them to erect protective fencing around the nest. If the nest is vulnerable to natural events such as high tides & floodwater then it should be left alone so that the swans can learn from the experience - if a young couple lose a nest under these circumstances then they will learn not to build a nest so low down the next year. Sad as it is, they have to be allowed to learn from natural experiences which is one reason why it is illegal to interfere with a swan's nest in any way."
I wonder if the nesting site in this case had been protected in any way - seems the council have some responsability for their protection....I expect that, like me, most people don't know this.
Lots more information on this site including the original Swan Law of Edward IV in 1482 which is still upheld to this day.
http://www.theswansanctuary.org.uk/index.php