Thread:Project:Support desk/problem editing Main page only

I have recently set up two Wikis on the same server and for one of these a problem has developed when trying to edit the Main page from the top level edit. I can edit all other pages and can edit the sub sections of the main page, but when I click the top level edit and try to save the Main page I see this

'Forbidden

You don't have permission to access /wikiwiki/index.php on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.'

I don't think there is a problem with permissions - I have all enabled and am administrator and bureaucrat etc, plus it works with the same set up for my other wiki.

Any ideas how to trap this ErrorDocument 404 error?