I managed to figure out what the problem is and I see you have given me very good advice in your replies. I had some geometry far away from the model I was working on. It appeared when I imported a dwg file into SU and it was so far along the vertical axis I didn't know it was there. When i zoomed out and deleted it the shadows worked perfectly again, no clipping. Obviously focal length had nothing to do with my problems.
Thanks very much everyone