One thing I've found is that on a night that is moonlit,but,has a fair bit of cloud,and is windy to the point that the clouds a shooting across the sky,giving you moonlight every few seconds does seem to give Charlie a false sense of security(that's my findings anyway).
I've shot plenty of foxes under all types of conditions though,but,as John said,if they are educated to the lamp then you will have trouble,and,you need to use your best calling techniques to get a result.
Good luck D.S. and keep us informed as to how you get on mate.......Martin.