I'm probably doing something stupid but it seems to be autoloading my helper class

Thanks for the reply. To clarify - Are you saying that whenever you add the word "my" to a helper class that is already in the codeigniter framework it overrides it? If that's true then I'll just change the name of my class. I've now changed the name of "my_url_helper" to "assets_url_helper" and that has fixed the problem. Thanks again!