A couple of issues here -- there are some fairly intricate certificate/PVT key operations which while perfectly acceptable for plink/pageant may be going haywire. The SVN on the server can be perfectly OK and viable, yet the TortoiseSVN hookup craps out gloriously.
In theory, if the puTTY ssh config is working, you should be able to hook up via the inclusion of a 'load ' on the TortoisePlink command line you drop into the Settings->Network->SSH Shell entry. Notice the word 'should'.
I've tried every single step in every HowTo/FAQ/TortoiseSVN blurb out there (save some that require root privileges on the server in order to tamper with the /etc/svnserver parameters), and so far absolutely nothing works. The end result of about 2-1/2 weeks of effort is the message "Unable to connect to a repository at URL ...". Yet the SVN environment works like a dream, plink and putty work perfectly, and the SVN root path I pulled directly from the svn info line (minus the extraneous file:/// prefix).
I'm convinced there is something wrong that is so obvious that no one has thought to mention it in the literature.