The error occurs when I select Unenroll User From Course for External Unenroll Action which of course is the option that I really want!
Sorry, I thought, seeing as you were the LDAP expert, you must surely be running it!
So, I turned debugging on and when I ran it again, I also got an error referring to the social wall format. Uninstalled it and the ldap enrolment sync ran perfectly! I hadn't thought debugging would put extra messages in cli - thanks for the hint.
All good now. Well, will move to Social Wall format issue! Thanks for the help!