The prefix error might be a problem with the source data. But I need to examine your database to know for sure. I can take a look if you are comfortable giving me access to your database. I need access to phpmyadmin or a copy of your database.
Unfortunately, I was in click on through mode so I didn't note it down , next time I will.
I tried to fix the problem by trying to delete any 'unusual' characters from the prefixes, this didn't work, so I deleted all of the prefixes, which worked. It's a bit of a pain but not the end of the world as we only use prefixes in a couple of not that busy forums, so thanks for the offer Jake but it won't be necessary.
The rest of the import (800,000 posts and 55,000 users) went pretty well. Now to see what bits are different/missing from xf and see how/if I can add them