I solved the problem.
It turns out our live database had the default collation set to a different unicode set from our mdl_user table. This meant than when the LDAP sync script created a temporary table, it couldn't join it with the table in Moodle to see who needs to be added. I've now changed the default collation to match that of mdl_user, and all is well!
Enrolment
LDAP not syncing new accounts
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