Limiting access to an application

I have a site, call it H.com for the sake of argument (it is internal, so I
can't give a working link). This runs a CMS, https://H.com takes you to
this.

On the same box I run websvn, the web front end to Subversion, the URL for
this is https://H.com/websvn. It is usually called from within the CMS. The
directory is protected by Apache basic authentication.

What I would like to do (and have singularly failed at so far) is to prevent
the user getting to websvn except through the CMS. I thought that this
might be possible by using mod_rewrite, sending the user back to the start
page of the CMS if the referrer wasn't the CMS. However, I obviously don't
have the correct rewrite conditions.

Could anyone either suggest a better way of doing this, or give me a clue as
to what the rewrite conditions should be.

--
Colin Walls
Remove the pink meat to mail me