RoryBubble, one issue is that people have trouble helping for the same reason that the computer has trouble helping - we need a very clear statement of the problem before we can even start on a solution.
So the key point is
It cannot alter the links on your pages. The first question is this: is your Joomla site linking to the URLs that you want users to "see" and "use"? That's the first step.
Your reply to Andy_Langton says
They currently work, yes. But /streams/ is still there.
Where is "there"? - in the address bar URL after the page loads? - in the HTML code on the page?
It still is not at all clear to me what you're trying to accomplish, but it sounds to me based on your answer that your rewrites are working fine, what's not working fine is Joomla.
So this is how I understand the situation based on your reply to Andy_Langton:
1. These URLs currently "work", meaning that if I type them into the address bar in the form below, I - a) see the correct content - b) see them unchanged and in the form below in the address bar after the page loads
2. The URLs on your site - in the navigation and everything - are in the form
If that's the case, there's no mod_rewrite fu in the world that can correct that problem in a .htaccess file. You need to correct your output on the Joomla end.
If that is not the case, I'm sorry, but I'm just not following your answers.