Bug#111239: Further information....
Okay, I managed to fix the problem by flushing the cache on my proxy server
and on the installation machine, so it fetched the packages fresh from the
server...this time with no errors. However, the problem with it complaining
about the proc filesystem already being mounted, when running 'Install Base
System' multiple times after downloading failure, still exists and stops the
packages being configured. The general point (and I maintain that it's not a
error more a usability issue (and a difficult one to resolve at that)) was
to make the system a little more resilient (e.g.. resuming downloads would
be nice) and not for it to give up at the slightest error.