Thanks for the clarification it's a nightmare. I manually copied and pasted from a server running the backup copy of the DB, into the live site. It broke attribution for the posts (nearly 400 of them) but at least the post contents are back.
Would it not work to run the backup database on a copy/test installation then create a new forum and move that thread to it, delete all threads and forums except that thread and forum, delete all users not in that thread and then do an import?
I was able to restore a thread recently with a crude method.
Do this in your older database. Go to phpMyAdmin
1. Select the table xf_thread (xf is my prefix)
2. Find the thread_id of the thread you deleted.
3. Delete all threads except the thread you want. Run this: DELETE * FROM xf_thread where thread_id !=your thread number
4. Do the same for xf_posts. DELETE * FROM xf_post where thread_id !=your thread number
5. Delete all tables except xf_thread and xf_post
6. Now export the database and import into your current database