Mark,
I am no expert but think about the type of sheep you have and how good your fencing is. I have Ryelands and they just don't seem to do fence jumping. My tup is in the field next to my girls with four bar fencing in between. They often sit either side of the fence but there is never any attempt to jump over.
On the other hand I have seen the commercial sheep round here jumping fences as if they are training for the olympics, clearing it easily.