I have never had to jump the fence - just open the gate, and make sure it is shut behind you.

I added this to the field notes prior to posting last night:

http://www.themodernantiquarian.com/site/1775
Every time I have been here, there has never been anyone in at the farmhouse nearby, and usually both gates are padlocked. It is good to ask, but in my six or so trips here, I have never found anyone in. Furthermore, there is a big sign in one of the windows informing of the security of the building, so I reckon it is often empty for long periods. But by all means, try to ask.

Tim