[Exim] Forcing TLS in LDAP lookups

Does anyone know how to force StartTLS for Exim LDAP lookups, ie.
encryption over port 389. ldaps appears to talk SSL over port 636.

Dave
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Dave Lewney
Principal Systems Programmer, IT Services
University of Sussex, Brighton BN1 9QJ. Tel: 01273 678354 Fax: 01273 271956