How difficult is it to create a custom 401 page in apache while requiring basic auth for the web root. I cant work out how to allow just the file /401.php

I keep getting:

Additionally, a 401 Unauthorized error was encountered while trying to use an ErrorDocument to handle the request.

Any suggestions?

I've tried the following

ErrorDocument 401 /401.php
<Directory "/var/www/glype">
AuthType Basic
AuthName "Site Under Construction - Dev Only"
AuthUserFile /etc/apache2/.htpasswd
Require valid-user
</Directory>
<Files "/var/www/glype/401.php">
order Deny,Allow
Allow from all
</Files>

What am I doing wrong