Good question. As with any wildlife photography, knowing your subject is key. If you're seeing foxes at night, I'd start by recording some observations -
Where and when. Foxes are territorial and will usually range over the same area unless some outside influence causes that to change (more on that later). If you're seeing a fox at roughly the same time and in the same place, it's highly likely to be the same one. Look for different fur patterns, is it a dog or vixen? (might be hard to tell from a distance)
If you know you're going to be likely to see a fox, you're halfway to getting a photo!
Time of year will make a difference.
January-February is mating season, where they're intensely vocal and then the vixen will go looking out for a breeding earth
March-April, cubs will be born and will be underground, so any foxes you see are likely to be fathers bringing food to the vixen, or non-breeding females
May-August, cubs will come above ground, grow up and parents will be bringing them food.
September-November, cubs will be properly adult size and the family group will start to break up. That's when you'll get territories shifting, as the cubs will be looking to find their own place. December, they'll start getting more vocal as mating season approaches.
So, photography then. Foxes have good hearing and sense of smell, but pretty poor eyesight. So leave the aftershave at home :)
Which comes back to knowing where to find your foxes. I'd tend to avoid flash, as they'll get spooked. The urban landscape is rarely completely dark, so I'd go highest ISO you can, longest lens and tripod. Or try going wider, putting the animal small in the frame for context to it's surroundings.
See if you can catch one walking through a pool of light cast by a streetlight. You could try baiting particular spots with peanut butter sandwiches (they love them!)
Persevere, good luck and I'd be happy to join you :)