Thanks for that video suggestion. I've had a good look and tried some of the ideas suggested. However the chap explains that his students' sticks are slipping because they aren't bouncing their sticks back up so that they're parallel with the ground: ie. not doing big enough strokes.
My problem seems to be that when I am doing bigger strokes, the force of the motion is pulling my stick out of the front of my hand.

Thanks for the suggestion anyway.