But it wasn't my fox, it was another one that seems to avoid the dish of people food but will take the dog biscuits when I'm not looking. I could just barely see it on the river bank watching, so turned out all the lights and sat in the dark for a long time until it came back and grabbed some biscuits and raced off. I didn't see any difference from my fox in the brief glimpse I got, but there's no way my fox would act so spooked.
I hope this is a female because if it's a male there would be territorial issues. I think might be anyway, because the males usually hang around with the family until the babies are old enough and then go back to their own territory. But maybe the rules are slackened in winter for survival purposes and spring when a young fox's fancy turns to thoughts of love.
A couple more dog biscuits later, I got a better look at it against the snow. Remember that James Bond thing - All Cats are Gray? All foxes look alike when they are trotting across the snow in the pre-dawn. This one seems to be smaller, and definitely not used to people.
And then I got a really good look at it - my fox has a notched ear, and this one doesn't, so now we know. In general, this one is darker, smaller, and has shorter legs. Its face is darker around the eyes and has a different expression.
Sorry it's blurry, this fox is not going to pose for the camera and in the low morning light, my camera isn't fast enough.

Here is a picture of my usual fox for comparison:
