Graham Just a thought but I think you should redirect to a page that does not exist so that the proper error code is set
for example
<script>window.location.replace("/404");</script> is a redirect to an existing page so 404 is not set
<script>window.location.replace("/404.html");</script> is a redirect to a page that does not exist so xtgems 404 system takes over and sets the correct error code and loads the custom 404 page