Brilliant: resetting the system clock to 1st September 2011 worked.
Replaced my hard drive & reinstalled CS5 from original disk but had the same problem with updating as outlined above. Started manual downloads and installing patch by patch but when there was no English version of Photoshop 12.0.4 it tried this trick and bingo. Also installed the Pixel Bender for Flash app that I hadn't yet found.
Thanks, wish I'd saved some time and done it from the start.