We are in the city. No shotguns allowed. Only air rifle, but that is not allowed on foxes
ah. that's different then... you didn't mention that in the OP.
Hugh FW tried an experiment at the 1st river cottage (where he has chicken houses on stilts) where he got human hair from the local barber and hung bunches in stockings around the chicken area - the theory being that foxes hate human smell. I suspect this would not work as well in the city though.
One thing is for sure, the fox now knows you have a food source. you could try and trap it in a livetrap... then drive 50 miles away into the wilderness and do as you will (shoot/release). I suspect the local animal rescue types probably do this occasionally (they certainly did it in London), but I doubt they will do it by request... you could always ask them.
Yes, mulling it over some more I'd recommend buying a fox trap by mail order, get up very very early in the morning to check it, and drive it to the country. Otherwise the fox is coming back for your chickens.