In what circumstances will an independent contractor be held liable in a personal injury action for changing a piece of plywood in the middle panel?

In the Bates v. Parker case the facts are stated in the head-note thus [All E.R.]: "The plaintiff was employed by the defendant as an independent contractor to clean the windows of his hairdressing shop every fortnight. When cleaning a window with three panels, in order to steady himself, he had made a practice of putting his hand through a hole in a piece of plywood in the middle panel, which was bolted to the sides of the window. The defendant had seen him cleaning the window, but was not found to have known that he used the plywood in this way. A few days before the windows were due to be cleaned the defendant, intending to glaze the space occupied by the plywood, unbolted it and cut a piece of glass to fit the space, but, being temporarily unable to obtain putty, replaced the plywood in the panel without bolting it. When the plaintiff, while next cleaning the window, took hold of the panel, it moved and he fell and was injured. In an action by the plaintiff for damages for negligence against the defendant, the judge held that the defendant knew of the unusual danger due to the alteration of the plywood and held him liable."