Display different page on protected forums?

Hi all,

What I am after doing is this:

I have one particular forum that I was going to password protect. I looked at the hack on here but it has a bug in it whereby you can bypass the password by hitting refresh so I am going to create a different level of protection based on usergroup ids. Now, what I need to start it off is when a user who is not in a valid usergroup accesses this forum instead of displaying the threads it displays another page from a template stating they need to fill in a form to have access to this forum.

Where is the code I need to edit to check for a particualr forumid and check for a valid usergroup?

thanks in advance