Ok, Just in case anyone else searches for this.
I exported the user list from AT_members table and imported it into my new AT install, all appears to be working ok.
Couple of point for others :
A direct MySQL export and import of the table data failed, so I had to export as CSV and import that.
This may have been down to using differant MySQL versions (althow both were 5 releases) or it could just have been the front end I used to export/import the data (export with phpmyadmin, import with webmin)
The other problem was that the member_id numbers in AT_members clashed, this was becasue I had added a couple of users to the new install to test it.
And the imported data had users with the same ID number, I got around this by just deleteing the test users I had created before importing my old user data.
One thing to watch for, is the course owner uses this ID number, so if you remove your test users, and import your old users, make sure you double check who owns the courses etc.
Matt