It must have been such a shock for you to find Sooty's tail like that. How awful.
It's very unusual for foxes to attack an adult healthy cat as they are at a disadvantage because cats have retractable claws and foxes don't usually want to risk injury. Of course if they have cubs then they would be protective of them, likewise if a cat had kittens they would be protective of them. The foxes in my own garden are scared of my cats and keep their distance.
I wonder if it actually was a fox? I only ask because last year my local paper ran a story with the headline, "Fox Savages Cat". They even had a quote from the local vet's receptionist who said how badly injured the cat was. Fortunately the cat made a full recovery. Anyone, seeing the headline would have just assumed that a fox actually did attack a cat and even killed it! It so happened that the cat had been taken to my own local vet's surgery, so I went in there and asked about the attack. It turned out that they had no evidence that it was a fox and that it could have been a dog! The newspaper went with the headline "Fox savages Cat", because it was much more sensational! A fox had been seen in the area but then again foxes are scavengers and if the cat had already been injured by a dog then the fox may well have smelled the blood and approached. And foxes are territorial so it would have been in the area anyway. Unfortunately, someone saw the fox and just assumed that it was the fox who must have attacked the cat. I guess no-one will ever know. I tend to think that cats are more in danger from dogs than foxes.
Either way it's still very sad. Poor little Sooty. rest in peace little one.