FYI: i have solved this to an extent, if anyone else needs to know:
in the 404 page have this:
$qs = explode(";", $HTTP_SERVER_VARS['QUERY_STRING']); //for IIS only
$qs[1] = str_replace("http://{$HTTP_SERVER_VARS['HTTP_HOST']}", "", $qs[1]); //cut out the server bit
$qs[0] will contain the error code (ie 404)
$qs[1] will give you the relative path (to the root) of the requested document
i hope this is helpful :-/
PS, if anyone has a better solution...