Problem seems to have been resolved. It turns out that my A2 sheet layer template was causing the problem. I can't for the life of me understand why, but as long as I had the words 'copyright 2010 by George Phillips. All rights reserved. Do not reprint without permission' located next to my title block, then Vectorworks wouldn't export to PDF?!?!?! Strange....
As soon as I redrafted my A2 sheet layer template and moved this wording to the bottom left of the sheet, then the problem disappeared. I guess I just found some glitch.... but I did waste three days getting to the bottom of the problem. The wierd thing was that this A2 sheet layer template exported to PDF fine from a tired old XP machine, but my new notebook wasn't playing the game!
Thanks for your help. I've found these forums really useful.