[Christiaan]

Solved, my problem was I needed to change <?php bb_get_header(); ?> to <?php get_header(); ?> in the bbPress theme files, but I also needed to move require_once('../wp-blog-header.php'); up to the top of my config file.

It would be useful if both these aspects were mentioned in the Functions section of https://bbpress.org/documentation/integration-with-wordpress/

Who can I contact to suggest these changes to that page?