Well I went and installed Ubuntu on a laptop and basically I imagine the corruptions are due to there not being enough room on these cards to handle the update.
Only 33MB left free on the ext4 partition!!
My master SD card is only 4GB but with the ext4 partition shrunk by 500MB to allow me to DD it across to another SD card and for it to work even if the new SD Card is slightly smaller.
I did read the stuff about needing a 4GB card but expected something to tell me that there wasn't enough room rather than going amok! ce la vie
The master SD card has ended up with quite a few files missing from the FAT32 partition which is why it doesn't boot at all.
But I've managed to copy off my /home/pi folder and since that has my git repositry with all my Scratch stuff - I should be OK.
I think I'll simply start with a fresh download and go through setting it up again - it'll keep me out of the pub for a few nights
PS The corruptions being caused by shortage of space is only conjecture on my part but I'm usually a lucky guesser when it comes to computers
Anyway -watch out if using 4GB card with reduced ext4 partition size