Let's put this nest site in some context.
Most, if not all of the views that have been supplied have been from the closest spot that the nest can be viewed from, so the images have been fairly consistent. This however, is the view from the nearby bridge, the nest is to the left of the picture just above the pipe that is encrusted with years of Pigeon output. If you look at the river you can see that if the shopping trolley was not there, the Coots would have absolutely nowhere else to nest - hence their persistence at this one doomed site.
You might also notice that there is no marginal habitat, and that the river wall is quite high, so there is very little access for anything that cannot swim or fly. The river flows from left to right, and shortly enters a system of dams and weirs which would be quite fatal to any human that happened to float through. The river can be quite fast flowing on occasions, is reasonably deep and very silty.
It is for the reasons above, and many more I am sure, that the trolley has been where it has for so long, and why (I am told) it will be there for a while yet. It's bloody dangerous to go wading about in. So I can be certain that the Coots have all summer to make as many nests as they please.
The picture also gives you an example of how industrialised, and therefore rubbish, parts of my patch are.
The river Wandle doing rubbish, quite literally.