On a recent BBC wildlife podcast, fox expert Professor Steve Harris, Bristol University stated that the average urban fox will kill a cat every 6 years, and that some 500 cats live in every fox territory. So the risk is tiny.
A Bristol City Council leaflet writen by Professor Harris gives the follwoing advice:
This is very rare; a survey in northwest Bristol, where foxes were particularly common, showed that they killed 0.7% of the cats each year and these were predominantly young kittens. This means your cat is far more likely to be run over, stray or die from a variety of other causes.
Foxes are only a little bit bigger than a cat (males average about 5.5 kilograms) and are equipped with a set of sharp teeth. Cats have an equally sharp set of teeth, plus some pretty unpleasant sharp claws. If a fox tackles a cat, it risks severe injuries and that is the last thing it wants. Every night a single fox will meet many, perhaps dozens of cats and most encounters are either indifferent or amicable.
Cats and foxes will usually ignore each other. However, some cats are aggressive animals and will go for a fox, sometimes to drive it away from their garden or food bowl. Usually a fox will flee but if this is not practical and particularly if it is cornered, it may defend itself against the cat. Then both animals may be injured.
Finally, although foxes live in family groups and meet up periodically to play or socialise, they hunt alone. So stories of “packs of foxes” roaming the streets killing pet cats are totally fictitious.
Above photo from Wiki Commons of fox and pet rabbit by Oosoom.