I'm only a young hunter but i would definitely not be shooting at it with a shotgun as it might not kill it and you would be breaking the law if you let it get away badly injured.
I would recomend purchasing a fox trap(box trap) and capture the fox in it. Bait it up with a bit of meat or something.
Once you have caught it in the trap, put a thick leather glove on and grab it then break it's neck.
It may take you a good few days or weeks to catch it.
Regards,
Alex