You could simply insert some javascript code in the page to have it auto load the index.php page... should be a piece of cake... just add the following lines in your custom error page.
Naturally, you should remember to replace the domain name with your own.
&Script Language=&JavaScript&&
&!--
setTimeout (&changePage()&, 0);
function changePage() {
self.parent.location= &http://www.yourdomain.com/&;
}
// --&
&/script&