electric fencing

If you haven't done what Hillfooter mentioned ie putting damp-proof membrane strip under the fence it is worth doing as it will stop the grass growing up, touching the fence and shorting it out. You just push your spike posts through it to hold it in place. Builders' merchants (even B&Q) supply it. Go for the widest strip  you can find. If not - on a  winters morning when the grass is long and damp and mr fox is particularly hungry - you might regret it

Edit : oh and if you want to advertise your  the effectiveness of your fence to local foxes, unwanted cats, just tie a few tiny strips of bacon on to it - very effective at getting the message across.