Could be that the camera mount has taken a slight knock too, and even the smallest micrometer of mount distortion affects the cameras ability to render the image sharp at the edges.

Obviously if you have other lenses to test with, especially good quality prime lenses, then at the least do this and assess if the camera has taken any impact.
Good advice. I purchased a new lens as this lens is the main work horse for my real estate photography business. The new lens is a sharp as a tack so it looks like the camera is ok. However if I can find someone to repair the damaged lens at a reasonable price I'd like to get it fixed as it would be good to have a backup lens. As an aside, I bought the new lens from Japan on Tuesday. It was at my door on Thursday. Why does it take so long to get items from Sydney?