Thanks for your help guys - I did finally find the cause of my problem:

http://sqlforums.windowsitpro.com/web/forum/messageview.aspx?catid=60&threadid=42001&enterthread=y

Basically this is the output you get when a query use a parallel QEP.

Hopefully that info will save someone else the grief and hours this has taken me to get to the bottom of!