PMP called me and explained how to do this properly ( and yes I should have backed up the DB before attempting a BIG import from AD - hey everybody is entitled to one bad day at the office ).
The solution is to ignore the OUs to import box altogether and click
Import (yup don't fill in any details other than the account to
use to discover the resources ) you'll then be able to pick an OU
from a list. Not entirely intuitive. It seems typing anything in the
OU box is likely to pull in everything it can!
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