HI Darren
As long as you don't cause any disturbance to the fox earth itself then there shouldn't be a problem.
I would advise carrying out this work as soon as possible as we are just coming up to the breeding period for foxes so if the work is put off for too long then there is the possibility of there being a pregnant vixen, or if left even longer, cubs in the earth & it would certainly be more risky at that point.
This advice is given assuming that you are in the UK.