As my memory serves me correctly have you thought about running an electric wire with you mains unit round the top of your fence? You can purchase insulators. Then the fence can be your first line of defence and the netting your second.
I would put up two wires on connected to your mains unit and the other to earth.
Why? Because the fox may have some protection from the mains unit as he/she is touching the fence there for it may not complete a circuit and giving them a shock.
Here is a picture of my run I have some
stand off electric fence insulators (the sticky out ones at the top) and some normal electric fence insulators at the top. The idea is that if the fox climbs up the mesh he/she may not get a shock when they get to the top, as they do not have contact with the ground, and therefore not making a circuit and getting a shock. So what I have done is put another line around the top and this wire is earthed, so in essence they would be touching the ground.
"For a secure boundary fence which will also exclude foxes, consider a 9 wire fence approximately 120cms (4ft) high, using stranded steel wire. With the fence wired alternatively live/earth a fox scrambling over or jumping between wires will receive a shock even though his feet are off the ground."
Taken from
http://www.rutland-electric-fencing.co.uk/PageAnimalSwine.aspx