Heya Matty,
The multisite code in here uses is_blog_user in a few places, which is deprecated (and thus spits out warnings in debug mode).
Here are the codex pages for is_blog_user, and its replacement, is_user_member_of_blog
http://codex.wordpress.org/WPMU_Functions/is_blog_user
http://codex.wordpress.org/WPMU_Functions/is_user_member_of_blog
Note that the new function, is_user_member_of_blog takes 2 parameters instead of just 1, so you'll need to instantiate a variable with the current user's ID and pass that into is_user_member_of_blog.
Cheers, mate!