It's hard not to love ducks and mostly this affection is completely harmless but sometimes an admirer can get a bit carried away. Foxes, for example, are notoriously bad at maintaining a balanced relationship with ducks and so it is best for all parties if they don't meet in the first place.
Luckily the Eglu is a very safe place for your ducks to be. The run is made out of steel weld mesh, much stronger than ordinary chicken wire and has a 15cm skirt to prevent digging. The eglu itself is made from a sturdy twin walled construction and has no exposed catches for the fox to open.
Precautions you can take
If you have an extremely persistant offender then you can call in a pest control company (look them up in the Yellow Pages).
At the time of writing councils are not taking the problem of urban foxes particularly seriously, so why not also write to your local MP to raise the problem.