Which setting on a standard HB505/506 Ifor Williams trailer do you set your breach bar at for a horse that stands around 15hh. My lowest seems a little too low, and the higher one possibly too high.
I would use the lower one, especially for that height horse. At the weekend I had to rescue a horse who had got himself under the breast bar of the trailer at Tweseldown. It was genuinely horrific. Personally I would think the horse would injure himself less going over than under from what I witnessed.