Can't see where you are going wrong but here is my conf I use to protect subversion:
Options FollowSymLinks
Order allow,deny
Allow from all
AuthLDAPAuthoritative on
AuthType Basic
AuthName "CNCS Domain Subversion Access"
AuthLDAPBindDN "CN=Apache LDAP,OU=Service Accounts,DC=cncs,DC=co,DC=uk"
AuthLDAPBindPassword
AuthLDAPURL "ldap://skoda.cncs.co.uk:389/OU=Users,OU=Administrators,DC=cncs,DC=co,DC=uk?sAMAccountName?sub?(objectClass=*)"