Specifically, make sure in httpd.conf you have ErrorDocument 404 /error.html (replace /error.html with the path to your error page). And make sure you restart Apache after making changes to httpd.conf (APACHE)
The last line is LIVE…..
# Some examples:
#ErrorDocument 500 "The server made a boo boo."
#ErrorDocument 404 /missing.html
#ErrorDocument 404 "/cgi-bin/missing_handler.pl"
#ErrorDocument 402 http://www.example.com/subscription_info.html
#
ErrorDocument 404 /404test.php
Plesk error documents location
D:\Plesk\VHOSTS\CODEHAVEN.CO.UK\error_docs