Added Waves autotune to a project I am working on. Sounds great when you press spacebar. Try to export the mix, however, and the damn thing only exports about 10 seconds of the mix. Am I doing something wrong or have I bought a plug-in which is a heap of ****?

Why are you pressing the space bar??? And what version of Waves Auto Tune are you using? I believe there are about 3 different versions, but all of them are designed to be used on one track at a time. Waves also has tutorial videos for all their plugins I highly recommend watching.
 I use Waves Tune Real-Time and never had the slightest problem.
Anyway Waves make some of the best plugins out there, so I doubt that's your problem.
 They will either work or they won't, and if they don't contact Waves support, give them remote access to your computer and they will fix it for you within a couple of minutes.
 
Sounds to me you are only exporting a 10 second 'loop region' of your project???
 
 Before trying to exporting chick on "Ctrl A" to select All Tracks.
Make sure the 'loop region' turn blue and end markers are yellow on top of the time line extends to the end of the song project then hit OK to export.
 

Why am i pressing the space bar? To make sure the sound i am hearing in CBB is good. and yes, the sound I hear is good. 

Nightmare is when i try to export the audio. The export bar shows progress and....DUMPH (that is the sound I hear when the export is fininished) I close CBB and again I hear DUMPH.

I load the exported audio into Sound forge and all I get is about 7 seconds of audio and then nothing. Would love to know what is happening...

Definitely get in touch with Waves support.  They're very responsive and will get you sorted out.

Sorted. The problem was with Dimension pro which for some reason wasn't loading. No idea why that is but i have deleted it from the project and everything is back to normal. Thanks to all those that replied to my previous query.