In case anyone else in the future is battling with this, I got a response from Matlab:
• The issue with startBackground is a known issue with NI SensorDAQ and the Vernier SensorDAQ. • The developers are working on this and the issue will be resolved in one of the future releases. • Currently there is no workaround for the issue
i.e. The output cannot be plotted as of now in real-time.
Unfortunately, the README document that comes with the package suggests you can use startBackground and this 'known' problem isn't mentioned anywhere... I've contacted matlab to ask them to update this, so hopefully this won't continue to be an issue.