My wife is using Thunderbird on a new[ish] xubuntu 14.04 system.  She
has now got it fairly well configured and working the way she wants so
the next step is to move her old 'local' E-Mail across.

No way is this remotely easy, I can sort of see how it might be done
but there are so many gotchas that I fear it's going to be a mess.

Why can't modern E-Mail program simply use mbox (or maildir, I'm not
too fussed) as they were designed rather than having to add all sorts
of specially named directories and indexes?