Here's the field landing website....
http://www.field-landings.co.uk/
Nothing to add to this discussion other than a lot of the gliding accident reports pertaining to field landings usually contain the phrase "late field selection".
One of the rules of thumb I was trained with was start looking for a field at 1500 to 2000ft, pick one by 1000ft and local soar it, and if you get to 700ft start a circuit and land. Altitudes all judged by eye as well. With a sink rate of 200 ft/min, that means you know where you are going to land about 5 mins before getting there. Never had to do this for real mind.....
Cheers
John