I was reading this thread yesterday and was reminded of a chapter in Alan "Bunny" Bansom's book, Be a Better Pilot, called "The Impossible Turn".
I looked at it last night and it's worth a read if you can get hold of a copy. His view was almost no matter what is ahead/to either side of you on take-off in a light aircraft, take it everytime over trying to turn back unless you are over 700ft. He supports this with various diagrams and equations which appear to show that it is indeed an impossible turn.