The law concerning health and safety at work is that all persons are equally responsible for their safety and that of others. You can't turn round if an accident happens in the circumstances you describe, and say you thought it was safe, cos it's patently not.
You could try asking your boss to show you how to do this job, or you could develop a really bad back or something that prevents you getting onto any such equipment. It will irritate buildersmate if he looks in on this post, but if you were in a union this would be a clear case fo their involvement.