htaccess 301 redirects

[Stevie GooGs]

I have an issue with my forum, everything is installed on .com but we have to point everything to .co.uk including the forum. When i add the redirects and go to the forum when i go to login it says thanks and redirects but it kicks me back to the forum like i have not logged in?

RewriteCond %{HTTP_HOST} ^www.mydomain.com [NC]
RewriteRule ^(.*)$ http://www.mydomain.co.uk/$1 [R=301,L]

If i change the vbsettings for forum to the .co.uk it does not work

If i go to the .com domain and login its fine?

What i want is the forum to just be .co.uk and users to be able to login.

If i manually put in

http://www.mydomain.co.uk/admincp/index.php and login it kicks me back to the same login page, if i replace the .co.uk with .com it works fine.