MacBook Pro and external monitor

Please bear with me if I'm missing something obvious ... but I'm out of ideas and settings to alter.

I have a presentation to do and as a trial run I have connected my new 13 inch MBP to an external monitor to mimic the projector to be used during my presentation. Connected by a VGA adaptor purchased with the MBP.

I'm using Buzan's iMindMap software for the presentation which presents in a similar way to Powerpoint etc in that the MBP displays a split screen showing what the audience sees and additional information for the presenter whilst the external monitor/projector displays only the main presentation view.

Problem occurs when I click to linked documents (pdf) within my presentation. On the MBP screen everything behaves as it should - pdf opens in Preview (although I have also used Adobe Acrobat) and I have a "Resume Presentation" option to go back into my iMindMap presentation - this works fine.

However, on the external monitor once I attempt to open a pdf document, ie, when I leave the iMindmap app to use a pdf viewer, the display becomes one of the default Mac desktop (the stars/asteroids/space image) with the monitor display settings window from within Systems Preferences. Is it relevant/helpful to point out that I do not use the default desktop image on my MBP and yet it appears on the additional monitor ?

Mirroring the two monitors works without a problem but that defeats the object of the presentation view, ie, the audience see the three split screens the presenter is using. I have emailed support for the iMindMap application.

Any useful insight/observations/suggestions gratefully received.