Fortunately there are lots of things you can do to improve the safety of your chickens.
Be concientious about locking the chickens up at night, preferably as soon as it gets dark.
Don't leave any food lying around the garden.
Make sure that the dustbin is always securely closed.
If there is a male in the household, ask him to urinate around the garden this will act as a chemical repellant to the fox (honestly!)
Tying up bags of human hair around the garden works in the same way.
If there is no male available or everyone in your household is bald, you can buy a chemical repellent.
Increase the height of your fence to at least six feet all the way round.
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.