von Mike Wilson.
Thanks Michael, that looks to be exactly what I'm after.
From the looks of things course _standard_elements is used for adding extra fields, coursemodule_validation for validation and course_module_post_actions for processing the form elements.
I've found a good example of the callbacks in action in admin/tool/lp/lib.php "Inject the competencies elements into all moodle module settings forms" which is what I'm looking to do.
Many thanks for the steer in the right direction