I had a different layout (around 3 miles long) planned, but then I saw it had to be a national circuit, so I scrapped that and made this one, now a bit under 2 miles long. Probably a bit too hairpin-y, and I don't suppose it works very well in either direction (I drew it going anticlockwise), but it's the best I can do given the area.
http://www.gmap-pedometer.com/?r=1090361