We’re all familiar with the black-and-white images from the 1920s, of construction workers in New York sitting on a beam, hundreds of meters from the ground, having their lunch. Not a hard hat or a safety rope between them. Today it would, quite rightly, be unthinkable. We’re never likely to see a repeat of this death-defying stunt, but if you employ people who work at heights, how do you keep them safe?