You can find Jason's site in the sponsors page, but here is a direct pointer; http://www.alternativecamera.com/index.html
The focal length video is here; http://www.youtube.com/watch?v=CIFNdfjem18
Many thanks for all your replies, you have been most kind & helpful!
I have found the focal length, as far as I can ascertain. It is 11 1/2 " (possibly 12" at a push). Have just got to find some 1/2 plate film now. Any ideas anyone??
Thanks again, Marcus