You must ensure that everyone involved in the work is competent (or, if being trained, is supervised by a competent person). This includes involvement in organisation, planning, supervision, and the supply and maintenance of equipment.
Where other precautions do not entirely eliminate the risk of a fall occurring, you must (as far as is reasonably practicable) train those who will be working at height how to avoid falling and how to avoid or minimise injury to themselves should they fall (see paragraph 20 of this HSE publication).