Know the feeling well, not with foxes but cats!!!
We tried everything, including lion dung, and they just kept coming back
Anyway, splashed out on one of those ultrasonic gizmos and hey presto no cat mess!!!
I can't see any reason why it won't work with foxes however there will be one draw back.......
The foxes won't come back to your garden
catch 22 I feel
Gadgetier.