Have you done giving to the bit, Miss Dent? That's pretty much the foundation for the one-rein stop. www.horsemanship101.com
or looking up John Lyons can help. He's really good at that sort of thing.
Just to be argumentative: isn't 'natural horsemanship' kind of contradictory? Because if you think about it, horses naturally would never want anything to do with people. What we do is completely unnatural: telling them to follow a predator, suppress their survival instinct and think out a situation, and be ridden easily. 'Natural' and 'horsemanship' seem rather oxymoronic to me.