I tend to do it when they are about 6-8 inches. This may be leaving it a bit late for some as pinching tall plants removes the growth that will eventually produce flowers. So the earlier you pinch (within reason) the sooner the plants will bush and therefore flower. They dont flower earlier because of the pinching, its the time they have to re gorw after being pinched that determines this!! HOpe that made sense!?

I thought that you pinched the shoots out once the flowers were fully out.....the reason for doing this, apart from bushing out the plant...was to deter black fly.and would help the pods grow?????

I certainly would not pinch out as low as 6 - 8 inches! How on earth would you get flowering shoots and pods? I've never experienced broad beans bushing out. We simply let ours grow until they are as tall as we want and then pinch out the tip.Has always worked for us...