Sorry to drag up an old thread, but I wanted to let people know how I solved the issue.
The tip above for selecting everything then creating a block worked, as it showed that I had something outside the model about 10km above the space I was working in. I was then able to select the items I wanted and group them. Then a elect all, and de-select the model and a delete got rid of what ever I'd managed to locate on the edge of the earths atmosphere. This got rid of the clipping plane and restored my sanity!
So thanks to the guys above and your tips!