Although I'm sure it's possible I cannot see how I would be able to do the following.
Using .htaccess to point the html errors to a single error handling page (this bit not a a problem).
That page then checks what the error code actually was (404, 500 etc.) and displays a message according to each one.
I am trying to do it in a PHP environment, and it's for a minimalistic site so the less files the better.
This is an example of a 404 I'm producing, http://isitup.org/404, and all I want is to switch the number accordingly.