Using banksmen to control reversing operations can put the Banksman in the potential danger area of a reversing vehicle. Every year banksmen suffer serious and fatal injuries whilst at work. If you do use banksmen, make sure they are trained to carry out their duties safely.
There must be a safe system of work that ensures the Banksman and driver are using standard signals, so that they are easily understood, and that the driver knows to stop the vehicle immediately if the Banksman disappears from view.
The standard signals are given in the Health and Safety (Signs and Signals) Regulations 1996 L64, available from HSE Books.