Hello Duncan,

Customised 404 error pages are dependent on access to your server, and not a feature of WebSite X5 as such.

Depending on who you host with you may find that there is an option in your online control panel to set one up with a minimum of effort.

Otherwise you can just create a standard webpage customised to your liking, and use the .htaccess file in the root of your server to point to it with a single line as follows:

ErrorDocument 404 /your_customised_pagename.html

There is no reason why you can't use X5 to create your customised page, which you can then set as a hidden page in the sitemap.  The page can have a .php extension if necessary.

It's a good idea to use a robots.txt file to exclude your 404 error page from Google's search index.

Hope this helps!

Paul