The first thing I tried was just renaming the file page.php to index.php. When I browsed directly to this file I got the same error I'm getting now. This was before I had changed any templates or scripts. Now I have changed all references to page.php that I can find, but I am still getting the same error on the homepage and category pages. The error isn't saying index.php doesn't exist it's saying the category doesn't exist.
James.