Trying to setup htacces for a website on our intranet but hitting some issues...

Main site is : http://portal/
That works fine....

Now Ive setup a Wikimedia install which is working fine but want to prevent access to it from everyone except a few of us. (http://portal/wiki/)

Ive created a .htpasswd file with the passwords etc and placed that in a directory called Private and then placed a .htaccess file into the Wiki directory.

----------.htaccess file---------
AuthName "Restricted Area"
AuthType Basic
AuthUserFile /private/.htpasswd
AuthGroupFile /dev/null
require valid-user

When I try this, it just gives me a 500 error.
If I try

AuthGroupFile /dev/null
AuthName "My Private Directory"
AuthType Basic

require valid-user


I get prompted for password, which I enter, then gives me error 500.
What am I doing wrong here ? :) (Apart from everything probably ;) )