Hi - thanks for your suggestions.
After seraching (again!) through the forums, I found this post
which helped to solve the problem.
The answer is to run (as root)
tune2fs -r 0 /dev/hda6
to set the reserved blocks to zero (as it was on my original partition).
Hope this helps others!
I can't see any advantage to having reserved blocks - if they're never used, they wont help reduce fragmentation.