Hi Sam,
I'm glad my "comprehensive answer" did help.
Since the questionnaire plugin is an "add-on", like all add-ons, it does not get automatically upgraded when you upgrade your moodle core files. You have to do it yourself, i.e. find out the latest version available for your add-on, and install it on your moodle site. It so happens that there is not yet a "stable" version of Questionnaire available in the Moodle plugins Directory for Moodle 2.4. In the meantime, you can either use the version for Moodle 2.3, which will work on a Moodle 2.4 site, OR you can use the current 2.4 version, which is available from our Github repository (which you have done).
All the best,
Joseph