Gents,
I had to make changes to the Thank you page, too. Here's how to do it:
1- click "plugins"
2- click "edit" under your plugin called "surveys"
3- on the right-hand side of the page click on "surveys/show_survey.php"
4- search for the "print t" statement that has the text you'd like to alter (in this case, "Thanks for taking the survey..." Whatever you change that to will show up in the future... Just be careful NOT to edit or add quote marks or other PHP symbols (you might break the plugin).
Just FYI: If you look lower in the file, you'll find form information that you can replace with your own custom form if you'd like...
Cool stuff!